Hmm...I see that there are some people who believe Snape is negative and some who believe Snape is positive. Well, I for one, believe that Snape is positive.
My reasons?
Because JK Rowling cannot make Snape bad just like that. Dumbledore had utter trust in Snape, and it's got to be because of a reason that he trusted him. Remember, before Snape killed Dumbledore, Dumbledore was saying "Please Severus, please"...there could be many meanings to that. But in my opinion that means "Please kill me"...remember when Harry was going for a lesson from Dumbledore, and Snape was saying "I can't do it"...maybe Dumbledore was telling Snape to kill him, and Snape was saying "No I can't do it".
It's just, too all of a sudden that Snape became evil. I don't think JK Rowling would do that.
And about the part where Snape told Voldemort about Harry's parents whereabouts, it was after that that Snape asked for forgiveness. I think yes Snape did love Lily, because she always stuck up for him. And it is possible that Snape told Voldemort not to kill Lily, but he did anyway. Perhaps because of that step, Snape turned his back on Voldemort. He only hates Harry because of James, but haven't you noticed that he's saved his life quite a few times, why? Maybe because of his feelings for Lily, who is well dead, but love never dies. If Snape loved Lily, then maybe it is that love that made him turn to Dumbledore's side and to ditch Voldemort.
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