After Voldemort kills Professor Dumbledore in Order of the Phoenix, Snape becomes the potions teacher and headmaster of Hogwarts. Despite his harsh and at time cruel character, readers have come to understand Snape's allegiance to Dumbledore, and his role as a double agent for the Order of the Phoenix.
In Deathly Hallows, however, Snape's loyalties once again become a mystery, as he willingly provides Voldemort information about Harry's location. Harry, believing Snape to be his enemy, chases after him when he sees him and Professor McGongall fleeing during the Battle of Hogwarts.
It is Nagini, Voldemort's snake, who kills Snape with a bite to his neck. While dying, Snape uses the last of his strength to conjure up the memories he shared with Harry. It is through these memories that Harry finally learns the truth about Snape's character, discovering that his loyalty was not to Voldemort, but to Lily and Dumbledore, and that his actions were driven by a tragic and unrequited love that ended with Lily's death.