Why is it so hard to understand that two things can be true at once: Snape was always going to become a Death Eater, even if Lily and he had stayed friends, however, at the same time, given a new chance to do it all over again, Snape would never have become a Death Eater.
Snape was a Death Eater--ie he was part of a murdering, torturing cult that targeted muggle-borns--that happened and it was his choice (though he'd been dealt a shitty hand to start off with). HOWEVER, when he changed, he truly did change to the point that even if he would have been able to keep Lily safe, he would not have joined the Death Eaters again.
J.K. Rowling: 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. J.K. Rowling: 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.
