So your campaigns involve destroying cults; in my game, "dark elves" are an elf subrace and "drow" are elves who are part of the Spider Queen's cult, with a large presence in the Underdark. Would you be willing to talk some about cult stuff, in the context of them being bad guys in D&D?

Sure! The angle that I come at cults in dnd is usually structuring them almost like a pyramid scheme - you’ve got a large base of fanatical followers who really commit to the Cult with every fiber of their being, and they’re led by a small group of (usually charismatic) leaders that are reaping the REAL benefits of the cult. These leaders are usually strong believers as well, but they’re the ones that like, the gods are actually talking to or who are making a fuck ton of money off the followers or anything like that. 

When you’re building a cult for dnd, I would recommend having Tiers of followers. 

• Low level: fanatical followers, worship both the god and the ground the charismatic leader(s) walk on, usually can be convinced to do whatever the leaders say. If you want to mix it up, you could have the party find one of the stragglers that has less faith and is basically being left behind by the cult if they’ve decided to cut their losses trying to bring them back into the fold or something. You’re going to have more fanatics, but also the people whose faith wavers in this tier. Have your cult followers performing rituals to keep them reminded how devoted they are to their cult on a daily basis!
  
• Mid level: Mid tier leaders, probably local priests with a decent paycheck of some kind from The Cult Leader. These people probably aren’t going to be persuaded to leave, since they’re reaping tangible rewards from being in the cult. You’re more likely to find stragglers in the low levels. These people are going to be charismatic, and devoted to their god/being/whatever out of their own self-interest as well as out of faith. They’ll also have a firm hold over the minds of their low-level followers, so they’ll probably be able to convince them to do a lot of things that they wouldn’t do otherwise. (Think like, Melisandre over Stannis Baratheon.) It would take a LOT to shake these guys from their faith.
• High level: The Cult Leader - could be the Actual God or the top level prophet of the god or something like that. This is your Final Boss, and they’re not dropping their cult leader status for ANYTHING. They’ve got complete control over the minds of the people in their following, they’re VERY charismatic and power hungry and will do anything to keep a hold over their followers, who are their power source (whether that’s literally, financially, metaphorically, it doesn’t matter, they feel they NEED to keep a hold of these people). Your party could certainly TRY to convince this person to just stop… but it takes a lot of commitment to build up a real cult with truly devoted followers, so it is incredibly unlikely.