Severus Snape was first a Death Eater. But after he leaked information to Voldemort about a prophecy that was later revealed to concern about Harry Potter, he realized his mistake. His true love, Lily Evans, would be killed along with James Potter. Snape tried to make Dumbledore to keep them safe, which worked. Dumbledore put the Potters under the Fidelius Charm. Their Secret Keeper would be Peter Pettigrew, who would later reveal their hiding place to Voldemort, other then sacrifice his life to his friends. Once Snape found out that Lily was killed, he changed sides and became a double agent for Voldemort and Dumbledore. Dumbledore trusted Snape after Snape showed his patronus. It was a doe, just like Lily's which meant he had feelings for her.