While I have constantly stated my beliefs that Snape is on the side of the Order and is most undoubtedly good, I thought I should at least write a theory for the other side. I think it'd be interesting to discuss the other side of the argument and to get your input and thoughts.
There are, of course, the obvious reasons:
1.) Snape killed Dumbledore with hatred on his face, and perhaps in his heart as well. Nobody can deny the fact that he used the Avada Kedavra, thus causing Dumbledore's sad death.
2.) Snape was attending Death Eater meetings and was keeping loyal to Voldemort. We could even assume that everything he told Bellatrix was correct, thus proving his true loyalties.
3.) Snape made the Unbreakable Vow with Narcissa and was helping Draco all throughout the school year. We don't even know what else he was doing to help Draco complete his task.
Other reasons include:
1.) Snape is good at Occlumency and would be able to fool Dumbledore of his truest intentions.
2.) Dumbledore said that when he makes mistakes, he makes
big mistakes. He could have made a mistake about trusting Snape, and this would be one huge mistake, ultimately casuing his own life.
3.) I posted before about a JKR interview where she said that the
redemptive streak in Snape was something we should look for and remember in the seventh book. I think a commenter said this but, Snape could be redeeming himself with Voldemort.
4.) Snape has always disliked Harry. What better way to complete it than to team up with his arch nemesis for a chance at getting him dead.
While I think these are all possible reasons for Snape's alliance to Voldemort, something is still telling me that he's good, and he'll prove it to us in the last book.
