Snape became a death eater because he wanted to become one and Voldemort in turn saw no reason to deny him 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. He was a genuine Death Eater for around 18 months, give or take, and then a fake one for years. Dumbledore, aware that Voldemort had ordered Draco to kill him, had asked Snape to kill him instead as a way of sparing the boy's soul and of preventing his own otherwise slow, painful death. Snape was Dumbledore's man from the moment Lily was murdered. Over the years he did whatever he could to try to make up for his past as a Death Eater, and he followed Dumbledore – for the most part – without question. But when the headmaster of Hogwarts revealed Harry's true fate, Snape's reaction was not unlike ours.
