Harry Potter and the Deathly Hallows – Part 2
Harry looks into Snape's memories and discovers that Snape was in love with Harry's mother and joined Dumbledore so that he could try to save her from Voldemort when he went to kill the Potter family when Harry was just a baby. It is also revealed that Snape was a spy and he and Dumbledore plotted for Snape to kill him. At the end of the memories, it is revealed that Harry is an "accidental Horcrux", meaning that a part of Voldemort lives inside him and that he must die.