To borrow a video game example, the cult of Skaen from Pillars of Eternity makes a lot of sense - the idea is, the worshipers of Skaen are abused servants, oppressed peasants and other such people who tend to get trod upon by their masters. So they all congregate and hatch elaborate, yet brutal plots to destroy them, guided by priests of Skaen motivating them to cruelty and revenge against those they deem to have committed crimes against decency.
Now, slaves seeking freedom from oppressors you'd put squarely into the Good category of things, the kind of stuff that a conscientious hero ought to be helping with. However, Skaenites in their powerless nature get drawn into an echo chamber for their revenge fantasies, the blood rituals they view as necessary to attain the necessary power to strike back at their enemies (and, it has to be said, are often conducted with willing, incredibly vengeful volunteers from within the cult) invariably change them for the worse, and their plots often bring a lot of innocents into the crossfire. And their god himself is just about the nastiest piece of work imaginable, just as you'd expect from a god of revenge plots. But being nasty pieces of work to spite their masters is what the abused servants want, seeing no hope in a good life for themselves (or in some cases hoping to bring about the downfall of a tyrant for the sake of their descendants).
Also, taking inspiration from real-life evil cults might be a good idea - for instance, they provide a sense of community and acceptance for the ostracized, grant power to the powerless, provide an alternative to a stifling prevailing ideology or perhaps the alternative in the region is simply worse (or is perceived as such).
Finally, you can also take the internet for inspiration - MRAs, gang stalking communities, certain kinds of subreddits, Stormfront, the deep web, all of these give an interesting perspective into what the inside of an evil cult might actually be like with their twisted little microcosms of disturbingly like-minded individuals.