When Snape kills Dumbledore at the end of Half Blood Prince, many people, myself included, felt unsettled. Why would Snape, who Dumbledore has trusted all these years, suddenly turn around and kill him? Even though Snape seems so evil we know Dumbledore must have trusted him for a good reason. And so here is pure speculation on whether or not Professor Snape has really turned over to the dark side. I present the evidence in favor of Snape's continued allegiance to Dumbledore:
* Dumbledore might have known he didn't have much longer to live anyway, and once he found out about Snape's Unbreakable Vow he probably told Snape he had to go through with it (hence the conversation Hagrid overheard).
* Dumbledore pleaded at the end of his life. "Severus... please..." Now, we all know that Dumbledore was not afraid of death (after all, to the well organized mind, death is but the next great adventure). It does not make sense that he would have pleaded. Dumbledore's pleading was really asking Snape to go through with the plan and kill him, and not back out at the last minute.
* The reason given for why Dumbledore had trusted Snape was pretty weak. Harry said it was because Snape felt sorry after he found out what he had done caused James and Lily's deaths. Dumbledore was too smart to only take this reason for Snape's turn-around, and it was only Harry's speculation anyway. There must have been another reason for Dumbledore's trust.
* Even as Snape is fleeing the grounds, he still finds the time to teach Harry: "Blocked again... until you learn to keep your mouth shut and your mind closed, Potter." He is reminding Harry the importance of Occlumency.
* Something else doesn't fit: in Order of the Phoenix, Dumbledore says "the eavesdropper was detected only a short way into the prophecy and thrown from the building." However, in Half Blood Prince Trelawney says to Harry that Dumbledore and herself were "rudely interrupted" by the eavesdropping Snape, then thrown out by the barman. Now, we know when Trelawney makes a "true" prediction she cannot be interrupted and would therefore go on with the prophecy. The conclusion here is that Snape heard the entire prophecy and willingly hid the second part of the prophecy from Voldemort.
* Observe the simliarities between these two passages:
"Hating himself, repulsed by what he was doing, Harry forced the goblet back toward Dumbledore's mouth..." (Harry is feeling hate and repulsion toward himself, but he is doing what he promised to do. I would guess his face shows hate and repulsion while he's doing it.)
"Snape gazed for a moment at Dumbledore, and there was revulsion and hatred etched in the hard lines of his face." (He feels hate and revulsion toward himself... and still he's doing what he promised to do.
* Dumbledore immobilized Harry. Many people assumed it was to keep Harry safe, but perhaps he did it so Harry would not interfer when he realized what Snape was about to do.
* Snape continually yelled that he was not a coward, which seems uncharacteristic of him. I think killing Dumbledore was the hardest thing he ever had to do, even though he knew it would be for the greater good in the long run. To hear Harry calling him weak after it was just too much.
* In chapter two, when Narcissa Malfoy makes Snape take the Unbreakable Vow, stating that he will kill Dumbledore if Draco fails to do it, Snape's hand twitched. This could be nothing, but JK Rowling thought it was worth mentioning.
* On page 180, Harry says that Snape "loves the Dark Arts." Hermione replies, "Well, I thought he sounded a bit like you." This is just another example of how Snape might be viewed one way but really feels another.
* In chapter two, Bella asks Snape many questions to determine if he is really still loyal to Voldemort. One of the questions she asks is: "Why did you stop the Dark Lord procuring the Sorcerer's Stone?" Snape says it's because he thought Quirrell wanted it for himself, and didn't realize Voldemort was involved. However, Snape is a highly skilled Legilimens, so wouldn't he have been able to find out about Voldemort by reading Quirrell's mind?
* Dumbledore had his black, injured hand, and insisted that he was only alive because of Snape's help. It seems that Snape knew about Dumbledore's attempts to steal the horcruxes. If Snape was really working for Voldemort he would have told Voldemort that, but Voldy seems to have no idea his horcruxes are disappearing.
* Snape had the perfect opportunity to kill Hermione and Luna in the dungeon, but instead he kept them out of harm's way. If he was truly a ruthless deatheater he wouldn't have cared about killing two more people- especially someone like Hermione, who he probably knows will be extremely helpful to Harry later on in his quest to kill Voldemort.
* In Goblet of Fire, when Dumbledore is trying to convince Fudge that Voldemort is really back in power, Snape rolls up his sleeve and shows Fudge the Dark Mark. If Snape was really under orders for Voldemort, he knew his master wanted to keep his return and secret and wouldn't have tried to prove otherwise.
* One theory for why Dumbledore trusted Snape so much: maybe when Snape repented Dumbledore forced Snape to make an Unbreakable Vow protecting Harry. In Order of the Phoenix Dumbledore mentions something to Harry about Snape protecting him. This would also be a good reason why Dumbledore would want Snape to live. Of course, if this did occur, it means there must be one other person who knows about it.