Severus Snape seemed like a complete lone nerd type from whatever details about him are in the books.
Why would he bother hanging out with future Death Eaters (Nott etc...) in Hogwarts when the downside was basically extra bad for him (Lily drifting away and not liking him) and the upside was slight (for Snape's personality type, hanging out with a clique isn't a great deal of an upside). He seemed too logical and analytical for such a poor decision.
Q: Lily detested Mulciber, Avery. If Snape really loved her, why didn't he sacrifice their company for [Lily's] sake?
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. 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.
Snape came from a loveless marriage. Hogwarts was his fantasy escape. However, from his first moment on the Hogwarts express, he is emotionally attacked by James Potter. His low self-esteem meets the reality that Hogwarts is no paradise. At the speech before the Sorting ceremony, Minerva McGonnigle tells the new students that their "House will be like their family". Hermione doesn't initially find this - possibly, neither does Severus. What he does experience is being attacked by James Potter and his friends (He later constantly refers to being attacked four against one). Where can he find allies, helpers and possibly friends - his house - Mulciber and Avery. Bad choice possibly, but Severus's immediate need is help and protection from James Potter and co. Moreover, James Potter NEVER ceases to bully Severus - Even Sirius Black admits this - "Because he exists". Is this any different from the Nazi or Deatheater attitude to Muggles?
In sum Snape becomes a deatheater because of James Potter and his bullying