As long as you Serve His interests, He sponsors you. Doesn't much care if you Serve in His name. - Obsid
That's why I brought up Ormazd/Ahura Mazda and Ahriman earlier. Because the White God has been associated with the monotheistic religions and portrayed as somewhat above the gods associated with polytheistic ones, the bad god of a dualist religion could be seen as having the same kind of above-ness. And might reward someone who a path based on believing evil nature to be truer nature (the type who thinks everyone wants to be just like them, but isn't honest and/or brave enough) - because it wants others to see those qualities result in power and reward, and so inclucate them. You're right, though, the believer would probably call it "beyond good and evil" or something rather than evil.
Then again, we've seen no indication that anything like Ahriman exists in the Dresdenverse anyway.
If you define the word "evil" only by public opinion, then you can think something is right and even good, despite being evil.
The original post's example of the baals fits into this category too. The term's been demonised in the popular consciousness, because it was used to refer to the gods of Canaan; but the baals weren't worshipped in Canaan because their worshippers thought they were demonically evil, it was the Israelites who (eventually) thought that.
Another angle, not necessarily based in evil, but probably more appropriate, is antinomianism. That is the transgression of prohibitions as a deliberate spiritual practice (for example some of the dualist Gnostic sects who believed the god of the Old Testament was an evil being from whose prison the God of the New Testament was trying to free humanity would deliberate break the laws of Leviticus as part of their worship). I don't see that kind of practice resulting in any kind of True Faith per se (it's more like a True Denial), but nor is it necessarily going to Sponsored by anything concrete, and there's some precedent for it in the Lawbreaking stunts, where breaking one of the Laws of Magic gets you some level of power towards doing it again (and gradually changes you spiritually).
So maybe the closest to a "true faith in evil" would be adapting the Lawbreaker Stunts so that you get something similar when you break certain implied laws (maybe the same as the Laws of Magic, maybe different, but based on the general theme of "don't screw with people just because you can") with deliberate metaphysical purpose - the
intent to make the act change your spirit, the faith that your spirit
should change in that way, because it's right and proper that a spirit should do so if it can. And it does, you become spiritually more adept at that action (the bonus you get), but your nature changes and becomes tied to that action (the rewriting of Aspects and loss of Refresh). Conversely, the people who do those things accidentally, or try to justify them and claim they don't matter, won't get the stunts because they lack the faith in the effect such an action should have.
Does this make any sense? (I'm basically writing it down as I think of it).