For sure, that's why I don't want to get too realistic. But I just wanted a step between "hanging out in town, totally normal" and "painting my face with the blood of sacrificed NPCs, worshiping a scary looking dead tree in the woods."If you want to hold on to that "you're completely justified in stabbing him" line, then it's because they're fanatical believers in whatever the cult is doing.
If you're looking for a realistic justification, it's basically the same reasons people do in the real world: a mix of belief, belonging (and the lack of it in the real world), drugs, coercion, and being sold a lie. The problem is that before long, the cult leaders of today are the next generation from the cult founders of yesterday, so there's a real good chance they're victims themselves (just longer ago, and so moreso). In which case, are you really justified in stabbing him?
Basically, as with many things D&D, it's probably a very good idea not to examine things too closely. Because look too closely and it very quickly can get very uncomfortable.
This is a good example of how a Vampire gets into a home as well, after draining the family nightly by calling them out into the night with hypnosis, or causing them violent fits all night by intruding on their dreams.Just as you are about to break from despair, someone approaches you, offering a cure. What choice do you have, how could you possibly say "no" at this point?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.