Answer Snape became a death eater because he wanted to become one and Voldemort in turn saw no reason to deny him his what he wanted. ... During the Hogwarts time, he is closed with some futuer Death Eaters in Slytherin, and Snape himself is very intrested in dark magic, so he is kind of born to be a Death Eater.
Explanation: He was a genuine Death Eater for around 18 months, give or take, and then a fake one for years.