People who are sick of Harry Potter and all the fuss about the books should skip this post. Also, this post contains spoilers, so look away if you don't want to be spoiled.

Theory #1: Snape is not the bad guy. Before you start pelting me with eggs, hear me out. DD asks for Snape as he and Harry are returning from the cave. I believe he does not not only because Snape is an accomplished potion maker (a very valid point by Sidrah), but also because Snape is a legilimus (sp?). DD knew about Draco all along, but did nothing to stop him and does not want him to become a murderer under duress of his parents being killed by Voldemort. DD is weak, but fearless in the face of the most feared werewolf of the age and several Death Eaters. Snape appears at last, and DD starts pleading. Pleading? Is this not the biggest clue that something is very, very off, especially since this is the person who complacently told Harry in the first book that death, to the well organized mind, is the next great adventure, and also, just as calmly, told Voldemort that there are things worse than death in this world? Why, oh why, would DD beg Snape to spare his life? Impossible and an insult to the man's memory to even consider it. Therefore, DD is pleading with Snape to do something else. He could either A) be asking him to do something they have previously spoken about or B) Snape could be reading his mind. In either case, DD can only be asking Snape to kill him. It is entirely possible that Snape, in the moment he looks down at DD in the tower, is reading his mind and gaining a summarized version of the theory of Horcruxes and Harry under the cloak and so on, and that DD is asking Snape to kill him. Note, in the concerned passage, that only when DD says "Severus, please", does Snape lift his wand and do the deed. Personally, I feel it more likely that DD is asking Snape to carry out what he has read in DD's mind (or decided previously, but we'll get to that in a minute), and Snape, his face full of (self)loathing and (self)contempt, finally does so. Why? Let's examine the two possibilities:

a) An extremely weakened Dumbledore, Snape and an invisible and immobile Harry, take on several death eaters, Draco Malfoy and a renownedly sadistic Werewolf. Do not forget the unbreakable curse would also immediately kill Snape, should he not assist Draco as promised, which would leave a fatally weak DD and a helpless Harry, should the cloak come off or one of the Death Eaters accidentally feel him on the floor. The result is not too hard to imagine.

b) Snape kills Dumbledore, thus cementing, in the eyes of the Death Eaters that he's fully committed to the Dark Lord. With one flick of his wand, Snape has put himself in a prime position to be the Dark Lord's most major confidante. As a spy, one can do better than that. Dumbledore, therefore, shrewdly, has placed Voldemort in the company of two wizards close to him who would help Harry, the first being Snape and the second being Wormtail who still owes Harry his life. Also, by having Snape do the deed, Dumbledore may want to preserve Draco's innocence (what little is left of it, after all, he is no murderer yet and does not seem to want to be, if not for the threat on his parents' life if he refuses).

Of course, we also know that Hargid overheard Snape and Dumbledore arguing. Snape may have, if he is smart, told Dumbledore at this point about having made the unbreakable promise to kill him if Draco fails to do so. If nothing else, Snape is very shrewd and has no lack of appreciation for the seriousness of his double role in this war.

Mind you, from the very beginning, DD insists that Harry is more important than he himself. This is why he does not let Harry drink the potion in the cave, nor cut his hand to give blood outside the cave. It is, after all, a fact that none but the Chosen One can vanquish Voldy for good and wise and powerful as DD may be, he is not the one and he knows it.

*phew* Alright, it's pretty late, so more odd theories will be expounded at a later date. If you have any contradictions, feel free to email.