|"I AM dark and evil! Really! Fear me!"|
Well, there are one of four possible explanations, presuming you don't just dismiss the concept of D&D "Good" vs "Evil" as the arrant bullshit it is:
* Did Hitler think he was evil? Did Stalin? Pol Pot? Almost surely not. Just because we have an OOC system mechanic -- or just because the winners write the histories -- that proclaims someone "evil" doesn't mean that they think of themselves that way.
* The dark gods will have their due. Failure to worship them will bring their anger down on the land, something that has been proven time and time again. The people in the pews might be trembling with fear, but they come nonetheless.
* They attract the losers, the misfits, the powerless, the people with nowhere else to go, those who crave vengeance. The dark gods are real, everyone knows that. If you can't beat the ones who oppress and bully you, worship at the altar of someone who can.
* Haven't we all seen decades worth of players commit all manner of bestial and violent acts, all in the ostensible name of "Lawful Good?" The light gods, they preach Good, and Truth, and Honor, and Love, but look at the depredations of their followers! Isn't it just a pack of lies after all? The dark gods, though ... sure, they might be "evil," and there might even be some justice to the charge, but at least they'll never lie to you. They're honest about who and what they are.