I've read all Harry Potter books, and in the end Snape is still a bad guy. DH SPOILER?

He definitely shows personal character in need of improvement. We see he holds on to childhood grudges indefinitely and transfers them to innocent children of his enemies. He is all for the dementer kiss being given to Sirius, possibly because he believed that Sirius was responsible for helping Voldemort find Lily. He ran with a bigoted crowd, embracing their ideology to some degree. A group that held negative views towards his best friend and love which is kind of like affiliating with neo-nazi's while having a Jewish friend. He is definitely not the most wholesome of characters, but you do get a sense that he has not embraced the death eater ideology. He seems genuinely upset by the death of the muggle studies teacher and other atrocities he can't stop without blowing his cover. He is also clearly under the sway of what Dumbledore considers the highest power: love. I think that is where his salvation lies. I think if Harry failed and the Death Eaters succeeded, he would eventually take matters directly into his own hands and try to avenge Lily himself. He would probably know his chance of success would be low, but would try anyway. I think he would have already if he thought he could get Voldemort in a vulnerable moment and know he would actually die and not just become a shadow figure again. Still, I think he would take the cowards approach if it were not for Lily. Lily is his best motivation for looking out for someone other then himself. He does take risks protecting the students at Hogwarts from the Carrows by "punishing" them by sending them into the forest with Hagrid (which he knows is no punishment), risking suspicion. However, it did take Lily's death for him to finally realize he needed to stop following peer pressure from his crowd. Without Lily, he probably would have eventually lost all his scruples and become as vile as Lucius.