"Snape also stopped Harry from using one of the Unforgivable Spells, don't forget. He also saved Harry from the Death Eaters."
Not only did Snape keep him from casting 'Unforgivable Spells', he did so in the manner of an instructor to a pupil. 'Keep your mouth closed and your mind clear Potter.' or something along those lines. He was not only showing how powerfull he was, but showing Harry how much he needed to learn in fighting against the dark arts. My mind is still open on Snape, but traitors and double agents are rarely ever considered heroes.
excuse me while I bump
I don't know if you've read the little companion book "Magical Creatures and Where to Find Them" but I was disappointed that there was no mention made of dementors. Nearly every other creature is listed, even those who only get a paragraph or two throughout the books. I didn't know that they could breed until this book. I thought they were some kind of a subversion of an existing creature, like Tolkien used. There is very little to go on.