I have lived a thousand years and will return and kill again and again until I kill my Majnun, and he rises anew from the slaughter.
~ SCP-1790

Leaders of cults, although not always intentionally evil, are often depicted as sinister; indeed, some notable real-life cult leaders have turned to the evil side.

In fiction, cult leaders may be gods or god wannabes, or simply religious fanatics. Think of them as a mix between a fanatic, a master manipulator, and an anarchist (the villain archetype, not the political ideal).