zealot - A fanatical person who is unyielding in their spiritual or political pursuits.
Juliet went into the hills alone to seek God, even though her friends called her a crazy zealot. She later testified that speaking to God in the storm was a profoundly healing experience.
vendetta - A blood feud in which the family of a murdered person seeks vengeance on the murderer or the murderer's family
As the final act of his vendetta against Rugen, Inigo pulled out his sword and said, "Hello. My name is Inigo Montoya. You killed my father. Prepare to die."