I just think that Severus was too special to be understood by the others. And as we all know people easier accept stereotypes than exeptions! Then I do not believe that a person who was so ''trully, madly, deeply'' in love with Lily Evans, could have been evil!
And then, Dumbledore the wisest wizard, he would have never trusted a person who didn't deserve it! So, I think he was the only one to see the best part of Snape.
Lily left him because she thought he was a deatheater, but I think, Severus only wanted to be different from James Potter,and didn't want to become a Voldemort's blind slave.....And when Lily was dead, Snape made the right choice, he had prooven to be loyal, brave and very valuable to Dumbledore, Hogwarts....and Harry.