Zealot

A zealot was a member of a Jewish sect which started in the 1st century. It later came to mean any extreme follower of a belief system.

In Masada, Judaea, a band of Jewish zealots were losing in a war against the Romans. Eleazar ben Jair, the leader of the group, devised a plan: they would all kill each other. When the TARDIS appeared there, Ian Chesterton tried to change history by helping the Jews fight, which the First Doctor counteracted by helping the Romans, trying to preserve time. In the end, Ian was chosen to kill everyone, and Eleazar killed himself. (PROSE: The Last Days)

The Thirteenth Doctor described a pirate Chief Sea Devil as a "zealot" when she learned that he intended to flood the entire Earth, committing genocide on humanity and all other air-breathing creatures. She contrasted this with the more "honourable" outlook of typical Sea Devils. (TV: Legend of the Sea Devils)

During the French Revolution in 1794, the "Tyrant of France" Maximilien Robespierre was both a zealot and a psychopath. (TV: The Reign of Terror)