It's important that your fanatics have an ethos, outside random mayhem and a love of evil for its own sake. Why do the cannibals cannibalize (especially considering how difficult humans would be as prey)? What benefits do the evil cultists get from their worship of a monstrous deity (both real and imagined)? They'll have religious guidelines like any other sect and would be unlikely (if realism enters into it) to be hedonistic party-animals and effective members of their religions at the same time, because most religions have rules and objectives, even the orgiastic ones, and successful religions with staying power would need to provide more than just excess or hate or vengeance to their acolytes, they need to offer some semblance of spirituality above all of that.

Same with any extremity- belonging to a "kill unbelievers" religion still comes with a sense of community or tribalism (often in conjunction with an "us vs them" mentality), and strict, austere legalisms believe that unwavering adherence to and faith in their bylaws and articles constitute deference to the divine. "Evil" religions can be fun, but faceless-goon religions of meaningless evil get tiresome very quickly.