Good point about Percy! I agree that seems very likely.

I disagree (of course)! Just because Snape is horrible, that doesn't mean he's evil. Rowling has gone to a lot of trouble throughout the series to make it clear that you can be on the side of good and still not be a very nice person and vice versa - in fact, half the plot of the first book is based around the fact that Harry suspects Snape of being evil simply because, in Quirrel's words, 'he does seem the type, doesn't he?' Just like any good writer, JKR has set out her stall on that one in the very first book, and maintained the theme since.

Bear in mind also that we only ever see Snape from Harry's point of view, a character who is predisposed to dislike Snape from the outset (he's a Slytherin! My God!) - with one notable exception. I think the Unbreakable Vow scene with Narcissa at the beginning of HBP is very revealing - it shows Snape's compassion and loyalty, two qualities which I think are very important in understanding his later actions in the book, especially in the scene with Dumbledore on top of the tower.

Snape is also playing the role of the classic spy or double agent. He is constantly maintaining an appearance, living completely as though he were what he pretends to be. This is true whichever side you think he's on, and it means we virtually never get to see the thoughts or feelings of the real Snape. Far from portraying a clearly evil or clearly good character, JKR has done very well in portraying a character who could be either - we won't know till the end.

There are any number of essays that attempt to demonstrate Snape is a good guy. There's a brand new one here, from the Harry Potter essays blog, and Red Hen's essay on the subject is thorough and erudite.

Mrs G
The Garner who cares.