Insofar as we are talking about the kinds of cults the PCs won't have guilt about slaying and bashing their way through, I tend to consider cruelty and hunger for power as the primary motivators. There are lots of reasons someone might join A cult, but the reason people join THIS cult is purely cruel and selfish. They are small, wicked people who wish to gain wealth, influence or power without putting in the work, and are happy to see innocent people pay the price for them. Cults of this sort worship beings of greed and pain and domination.
One thing I try to avoid is the real world evil of many cults directed at women and children. Nobody needs that in their elf game.