Many people believe, after the "Half-Blood Price", that Snape is a bad guy, and he has betrayed Dumbledore. Everyone believes that Snape is now a Death-Eater. However, I think that I know better. I think that Snape is still on the good side, and that he is innocent, and that he killed Dumbledore on Dumbledore's orders. I believe this because of the following. Before I start explaining, keep in mind that everything you see in quotations is what we know happened in the book, and what isn't is my guess of what happened:

"Dumbledore tells Snape after the dark Lord appears to go to the Dark Lord and visit him. Snape goes as told, and purely because of his loyalty to Dumbledore."

When Snape comes back, and tells him the Dark Lord's plan to kill Harry Potter, and how he is planning to take over the magical world. Dumbeldore tells him to do whatever it takes to keep Harry safe, even if it means sacrificing himself.

Somewhere between book 4 and book 6, Voldemort tells Snape about his plan to kill Dumbledore. Snape, of course, would have transmitted the message to Dumbledore, who would have told him to act as a Dark Lord's supporter, and do whatever it takes to remain on the Dark Lord's good side.

"In the beggining to Book 6, Snape is visited by Bellatrix Lestrange and Narcissa Malfoy. After they tell Snape about the Dark Lord's plan of using Draco to kill Dumbledore, he tells them it is no news to them. Narcissa asks Snape to perform the Unbreakable Vow so that she can assure that he watches over Draco, and if Draco fails, she asks Snape to kill Dumbledore himself. Snape, at the time, would have had second thoughts, but remembering him promise to Dumbledore, he probably makes the Unbreakable Vow, so that he can remain as a spy."

Snape probably tells Dumbledore about the Unbreakable Vow, and Dumbledore probably makes him promise to kill him when it is time. This is because Dumbledore cares about Harry's safety more than his. Snape swears to finish off Dumbledore to Dumbledore himself.

"Snape offers to help Draco, just like he promises Narcissa. In the end of Book 6, Dumbledore is killed by Snape, after Draco is too cowardly to do so."

Snape did what Dumbledore asked him to do, and what Voldemort asked him to do at the same time. However, his loyalty remains to Dumbledore, as it always be.

We already have a thread on Snape ("*Spoilers* about Snape"), so I'm moving this post to that thread and locking this one.