The 4e Dawn War pantheon handled this pretty well. Providing explanations for why mortals might pray to Bane or Tiamat and what they get from it. I'm not as familiar with other D&D myths, but you could take a similar bent.Furthermore-- whether it be an evil god or a demon, it needs to be able to grant its followers something of meaning and value. Yet, all too often, it feels within D&D that the followers gain nothing and instead simply get disfigured or infected or have their freedoms or abilities curtailed. If that's what's happening, then why on earth is anyone following such a god, or demons for that matter?