A central mystery of the series concerns Snape 's loyalties. For much of the series, Snape 's actions seem to serve Harry 's nemesis, Lord Voldemort, although Harry 's mentor, Dumbledore insists that he trusts Snape. Snape dies at the hands of Voldemort in the final chapters of the seventh book, at which time his back story is revealed: as a teenager, Snape was a close friend of Harry 's mother Lily, and he turned on Voldemort after the latter sought to kill Lily and her family.
when do you find out snape is good
in the final chapters of the seventh book