Snape’s tragedy

Well, that is Snape’s tragedy. Given his time over again he would not have become a Death Eater, but like many insecure, vulnerable people (like Wormtail) he craved membership of something big and powerful, something impressive. He wanted Lily and he wanted Mulciber too. He never really understood Lily’s aversion; he was so blinded by his attraction to the dark side he thought she would find him impressive if he became a real Death Eater.

— J. K. Rowling when asked why, if Snape truly loved Lily, he didn't give up associating with Dark Magic to be with her (BLC: Bloomsbury Live Chat)