Severus Snape was so nice to Lily and she was the only person to the know the good side of the Snape, maybe if he was so nice like that to everyone (not in an 'I love you way') but in a kind way he could of been a very good man. I don't want to say bad things about James Potter (Harry's Dad) but I think James brought out the bad in Severus, by teasing him, but as every good person knows you have to stand up for yourself, so he did in the only way he knew how (because of his parents) he reacted in a bad way. Now that Severus knew how he would be treated for the rest of his life at Hogwarts he wanted to fit in somehow in the only way he could. Into the gang of Slytherins.