Or, in other words, the refined version of the theories I've been expounding on for the past week or so, in one handy linkable place (for those of you who've been asking if they could link). Thanks to everyone who contributed suggestions, objections, and questions for me to answer -- the idea has been much improved and refined in the process.

THE (HOR)CRUX OF THE MATTER

In which it is proposed that the basin full of potion that Harry and Dumbledore found in the cave was itself a horcrux, and that the locket lying in the bottom of said basin was merely a red herring, planted there by Dumbledore to prevent Harry realizing what was really going on. It is further proposed that Dumbledore had been aware of this horcrux and its nature for years, and had long ago come to the conclusion that in order to destroy it he would have to drink it, necessitating that someone kill him in order to complete the destruction of said horcrux. It is finally proposed that Dumbledore was keeping Snape in reserve for this very task of killing him, and that in Book Seven Snape will be proven to have been acting solely on Dumbledore's orders with a view to Voldemort's ultimate defeat, and is therefore not guilty of murder or betrayal (in this case, anyway).

Thus the theory: now for the evidence. Page references, where included, are from the Raincoast/Bloomsbury edition.

Professor Dumbledore's Sense of Horcruxes

It may help us to recall that Dumbledore did not require Harry to extract Slughorn's full memory of his conversation with Tom Riddle in order to know that Voldemort had split his soul into a number of horcruxes, nor even in order to know what many of those horcruxes were. The fact that Dumbledore had already drawn accurate conclusions about these matters can be discerned from the pensieve memories he shows Harry, specifically highlighting the ways in which Voldemort obtained the locket, the ring, and the cup, before Harry even knows what a horcrux is. Clearly Dumbledore is leading Harry down a path of knowledge that he himself already trod a long time ago.

As such, the campaign of sending Harry to extract the true memory from Slughorn was for Harry's benefit far more than Dumbledore's. Dumbledore had already been looking for Voldemort's horcruxes for years -- in fact, as he says, ever since the events of CoS when he realized the Diary had been a horcrux and that there must be more of them if Voldemort were prepared to be so cavalier with the first one (p. 467). As he says, "I have been hoping for this piece of evidence [Slughorn's memory] for a very long time... it confirms the theory on which I have been working..."

So, if Dumbledore has been searching for the horcruxes for some time already, it is not unreasonable to conclude that he would have discovered some of them by now -- very possibly including the one in the cave. Note that although Dumbledore says in his conversation with Harry that he believes Voldemort did not succeed in obtaining anything of Gryffindor's to complete the set of relics belonging to the Founders (based on his statement that the sword Harry used to kill the basilisk in CoS is the only Gryffindor relic remaining and that it has always been kept safe from Voldemort) he does not make the same confident assertion about Ravenclaw. In fact, he says "I cannot answer for whether [Voldemort] ever managed to find anything of Ravenclaw's." But I cannot answer is not the same as I do not know. Is it possible that the strange substance in the basin was a potion or other magical liquid belonging to Rowena Ravenclaw, or at least that Dumbledore suspected it was?

If so, it is also possible that Dumbledore had been aware of the Ravenclaw liquid's existence and nature for some time, and come to the conclusion that if Voldemort had turned that particular relic into a horcrux, Dumbledore would have to drink it and then die (either by killing himself or being killed) in order for that particular horcrux to be destroyed. And note, even if Dumbledore did not know the precise location of the Ravenclaw relic, that would not prevent him from learning about it nor from making thorough plans to destroy it when it was found.

The idea that Dumbledore was previously aware of the potion's nature seems to be borne out by his behaviour when he and Harry find the basin. Indeed, even before they reach the island where the basin stands, Dumbledore has already made plain to Harry that he needs Harry to obey his orders without hesitation or question no matter what those orders may be, and refuses to give up until he has Harry's word on the matter, which strongly indicates that Dumbledore knows or suspects that he will have to ask Harry to do something that Harry otherwise would not be willing to do. And when Dumbledore finally finds the potion, he seems to know an awful lot about its properties with very little preamble (something Harry puts down to his formidable powers, but could equally be due to thorough study and foreknowledge), and he also conjures a crystal goblet which he seems to have been keeping in reserve and which just happens to be sufficient to penetrate the barrier and allow him to drink. I am not sure that any old cup would have done the job in this case: it may well be that this goblet, too, was something of Rowena's that was meant to accompany the liquid in the basin.

The behavior of Dumbledore on drinking the liquid is also strongly suggestive of the idea that the liquid itself is a missing piece of Voldemort's soul, and not merely a poison or other painful deterrent. The text describes him as speaking "in a voice Harry did not recognize", and the things he cries out sound more like the words of a boy or a young man (such as Voldemort would have been when he created the horcruxes) than anything Dumbledore would say. Of course, phrases like "I don't want... don't make me ... don't like... want to stop... I don't want to... let me go... make it stop... I can't..." might be said by anyone in severe pain, even an elderly wizard, but what about the following:

"It's all my fault, all my fault... please make it stop, I know I did wrong, oh, please make it stop and I'll never, never again..."

"Don't hurt them, don't hurt them, please, please, it's my fault, hurt me instead..."

"I want to die, I want to die, make it stop, I want to die!"

"KILL ME!"

Is it possible that by putting these strange words and sentiments in Dumbledore's mouth (particularly things such as "I know I did wrong" and "Don't hurt them, please"), JKR means us to understand, or at least guess, that Dumbledore is speaking for someone else, that he has taken something inside him that is not his own? Dumbledore has told Harry before that Voldemort wished to rid himself of such troublesome human emotions and impulse as fear, guilt, concern for others, and most of all (as JKR has confirmed in an interview as recent as this week) the desire for death. Could the horcrux in the basin have contained, or at least represented in part, those particular elements of Voldemort's soul?

The Basin and the Locket

Of course, we are left with the question of what the locket was doing in the bottom of the basin, if the liquid Dumbledore drank was the real horcrux and he knew this to be the case all along. Allow me to suggest that Regulus A[lphard] Black, who had destroyed the locket horcrux shortly before his death back in 1980, was never anywhere near that cave, or that basin.

After all, if the locket was the cave's true treasure, Regulus would not only have had to be a wizard equal in power and stamina to Dumbledore (unlikely) but would have had to have an accomplice come with him to reach the island (something Dumbledore says would not be possible if the accomplice was a full-grown, fully accomplished wizard -- Harry only gets across because he has not come of age), and help/force him to drink down the basin's contents. Then, after drinking the contents of the basin and obtaining the locket horcrux from the bottom, Regulus would have had to somehow replace the basin's contents with a volume and character of liquid equal to the one he had drunk, which is sheer madness considering he could just as easily have left his fake locket and taunting note sitting in the bottom of the empty basin and still obtained the same effect. And then, of course, there would be the difficulty of getting back across the lake and out again with his accomplice so that he/they could destroy the horcrux -- a task which would also require tremendous power on the part of the potion-weakened Regulus (and we still haven't figured out how he'd get his wizardly accomplice in the boat). On the whole, it seems much more logically tenable that the locket and R.A.B.'s note were brought into the cave from some other location, and that Regulus (if it is Regulus we're talking about, and given that JKR has pointedly mentioned him in both OotP and this book, it does not seem unreasonable to assume that it is) was never in the cave at all.

What I'd like to propose instead is that the original, real Slytherin locket containing the horcrux had been placed in the care of Mrs. Black at Grimmauld Place, to her considerable pride, and displayed in her cabinet along with other dark relics of distinction. Very likely the late Mrs. Black did not realize the locket was a horcrux: but she wouldn't need to know its true nature in order to guard it fiercely. For that, Voldemort's mere word and the implication that he was trusting in her to keep it safe would be sufficient.

Regulus Black, however, learned of the locket's true nature and set out to destroy it. It is possible that Regulus was a protégé of Dumbledore's all along, always intended to act as a spy in the Death Eater camp -- a sort of predecessor of Severus Snape. But whether he was always Dumbledore's man or whether he joined the DE's sincerely at first and then regretted and repented of the decision, there came a point at which he determined to get rid of the locket horcrux (apparently under the misapprehension that there was only one horcrux and that destroying it would make Voldemort truly mortal again). As has been pointed out by others, the things we learn of Regulus in HBP -- including that Slughorn marked him out for special attention -- seem to indicate that he had a great deal more power and intelligence than Sirius gave him credit for in OotP.

So, Regulus breaks whatever protective spells lie on his mother's cabinet of dark objects, and steals the locket horcrux. In its place, he leaves a false locket -- one that could easily be mistaken for the real one at a cursory glance, if one were merely expecting to see some sort of locket there, but which would not stand up to close inspection -- with a taunting note inside telling Voldemort that he has taken the true horcrux and destroyed it.

Years later, on his quest for horcruxes, Dumbledore finds or is given the true Slytherin locket, and sees that its horcrux contents have been destroyed. On one of his visits to Grimmauld Place (which Regulus no doubt imagined would always be a Dark stronghold, thus making it a logical place to leave a note for Voldemort), Dumbledore also discovers the false locket sitting in the cabinet, and the note enclosed. He takes the fake and leaves the real one, now empty of its horcrux, in its place -- this is the heavy locket nobody could open which is found during the cleaning of Grimmauld Place in the opening chapters of OotP. (Another possibility is that Snape was in some way involved with Regulus, possibly even helping Regulus destroy the horcrux, and gave Dumbledore the Slytherin locket or advised him of its location after the fact.)

Now we have Dumbledore and Harry in the cave, and Dumbledore needs Harry to force-feed him the liquid in the basin. If Harry suspects that the liquid is the horcrux, he may also suspect that drinking it will destroy Dumbledore, in which case he will feel himself guilty of Dumbledore's death for forcing him to drink it (assuming he can even bring himself to do so). Better for Harry to think it a dangerous, painful, but not necessarily fatal potion that merely conceals the real horcrux beneath. So, as Dumbledore mutters unknown phrases over the potion and makes passes over it with his wand, he either Summons, or simply drops out of his sleeve, Regulus's false locket. Then he tells Harry that the liquid in the basin must be drunk to reach the horcrux, and that it may temporarily incapacitate him and/or cause him great pain to drink his way down to the bottom, but that Voldemort did not intend the potion to be fatal and that all will be well if Harry only carries out his part of the bargain.

Dumbledore and Snape

Having drunk the liquid horcrux and taken that part of Voldemort's sundered soul into his own body, Dumbledore returns with Harry to Hogwarts. Note that upon arrival the very first person he asks for is not Pomfrey (as we would expect if he wished for healing/treatment), nor McGonagall (as would be natural if he were concerned with the administration and welfare of the school), but Snape. In fact, he is positively insistent that Snape, and only Snape, can help him and that he must see Snape immediately.

Of course, it might be that Dumbledore expects Snape to have or be able to brew an antidote to the potion he drank out of the basin; this desperate urgency on Dumbledore's part in itself is not proof that the liquid was the horcrux and that he needs Snape to kill him in order to complete its destruction. But coupled with the argument Hagrid reported Snape and Dumbledore as having had earlier, in which Snape tried to get out of doing something Dumbledore had requested of him and Dumbledore was adamant that he must follow through, it sounds as though the two of them had a prior agreement of some sort that Snape, at least, found distasteful, and the idea that Dumbledore had appointed Snape to kill him once he drank the horcrux would certainly fit into this category.

When Dumbledore and Snape finally meet, Dumbledore's reaction is also suggestive. Seeing Snape burst onto the top of the tower, wand in hand, Dumbledore shows no fear or suspicion of him (which ties in with the frequently repeated idea that Dumbledore trusts Snape absolutely). He clearly does not fear that Snape has turned against him or will betray him. So why, then, does he say so softly, "Severus..." in what Harry describes as a pleading tone, and then "Severus... please...."? Harry sees Snape's face contorted with "revulsion and hatred", which he (of course) assumes is his attitude toward Dumbledore, but given the absolute nature of Dumbledore's trust in Snape, a trust whose basis he does not feel free to reveal to Harry or indeed anyone, but which he regards as beyond question (see pg. 513), is it not just as likely or more so that Snape's expression is one of revulsion and hatred at himself for having to kill Dumbledore in this way? After all, Harry has a very long history of misinterpreting people's expressions and tones of voice -- particularly where Snape is concerned -- and of assigning blame where it may or may not belong. As such his interpretation of the situation is suspect at best, and it may well be too hasty to leap to the obvious conclusion (as indeed everybody does, once Dumbledore is dead) that Snape was Ever So Evil all along, and only biding his chance to kill Dumbledore once he was helpless.

Going all the way back to the "Spinner's End" chapter at the beginning of the book, we find Snape shocking Bella by readily agreeing to take over Draco's task, regardless of the cost to himself, should Draco fail. It is interesting to note that when Snape draws the curtains and assumes his air of confidence with Narcissa, assuring her that the Dark Lord has informed him of his plans involving Draco, there is every likelihood that he in fact knows nothing of the plan and is merely bluffing. This is borne out by the fact that he does not supply or fill in any of the details of this plan, only makes vague allusions to it and waits for the Black sisters to enlighten him. Fortunately both Narcissa and Bella fall for it, supplying Snape with all the necessary details to figure out that Draco has been dispatched to kill Dumbledore. And although Snape hesitates visibly -- to the point that Bella taunts him -- when Narcissa begs him to watch over Draco and take his place if necessary, ultimately he does not resist taking the Unbreakable Vow. This would of course make sense if Snape truly hated Dumbledore, was every ounce the traitor to the Order that he portrays himself to Bella as being -- but it would make even more sense if Snape had already given Dumbledore his promise to kill him, and thus knew that making the Unbreakable Vow to do so would only strengthen his cover with Bella and the DE's as a whole, without committing him to do anything more than was already expected of him.

The fact that Dumbledore would give Snape the DADA position this year also bears out the idea that Dumbledore knew Snape would have to do something before the end of the year that would make it impossible for him to continue teaching at Hogwarts. Killing the Headmaster, in such a way that no one but Snape and Dumbledore knew the truth of what had happened or why, would certainly fall into that category.

Finally, the question of why Dumbledore trusts Snape, and why he did not tell McGonagall or any of the other faculty his reasons for trusting him (other than to continually affirm with them, as he has with Harry, that he does trust him and that he has good reason to do so) may well have to do with (much as I hate to say it) Lily Potter. Dumbledore tries to tell Harry that Snape was horrified when he found out that the Potters were going to be killed as a result of his reporting the prophecy to Voldemort -- "you have no idea of the remorse" and "the greatest regret of his life" are the phrases he uses. And of course, Harry doesn't believe him, because everybody knows (and Snape has reminded us frequently, including in this same book) that Snape hated James. When Harry later reports to this conversation to the Order, Remus has the same reaction, wondering how on earth Dumbledore could possibly fall for such a ruse when Snape's hatred for James was well known. But nobody in the book thinks about Lily, and what Snape's feelings may have been for her... pharnabazus has some very interesting thoughts on the possible connection between Snape and Lily here.

If Snape had been secretly in love with or even just had a close friendship with Lily Potter, if he had worked closely with her and helped her with her studies (much as Hermione has done for Ron and Harry throughout the HP books), and Dumbledore knew this even though no one else did, that might indeed go quite some way to explaining why he had no doubt of Snape's integrity... but also why he never disclosed his reasons to anyone, since that would be an unforgivable betrayal of Snape's private confidence. Harry of all people perhaps has the right to know (which would be why Dumbledore visibly struggles with the impulse to tell him before lapsing back to his customary, "I am sure. I trust Severus Snape completely" on pg. 513), but that will have to wait until Snape either confesses (unlikely) or Harry finds out the truth by some other means.

In any case, if Dumbledore's mysterious trust in Snape is borne out in Book Seven (and it seems very likely that it will be, given that we have had it reiterated in every single book of the series), then it follows that there was a good reason why Snape killed Dumbledore, and a means by which Snape's integrity and innocence (for killing a dying man who has made you swear to kill him in the interests of a higher cause and the saving of many other lives does not, I think, constitute murder in JKR's universe) may be ultimately vindicated. I believe that the idea of the liquid in the basin being the true horcrux -- possibly the Ravenclaw one (call it the Mirror of Rowena, if you like) -- would certainly cover all those bases, and account for the oddities in both Dumbledore and Snape's behaviour in a way that Harry's interpretation of events (namely, that Dumbledore drank the potion in vain only to find a fake horcrux, and that Snape is Ever So Evil) does not.