Well, who are we to mock someone based on his religion? There are probably noncrazy Voodoo practitioners, just like with any other faith. And sure, maybe he made the people recite a bastardized version of the Lord's Prayer with his own name inserted ("Our Doc, who art in the National Palace for life ...), but he's surely not the first dictator to do that.
Sometimes evil looks like a high school principal.
But after a heart attack plunged him into a nine-hour coma in 1959 that left him with massive brain damage, things kind of went downhill. He demanded that his temporary successor, Clement Barbot, be arrested, but when they couldn't find Barbot, Papa Doc's people told him that they believed he had transformed into a large black dog.
Understandably, Papa Doc ordered the deaths of all black dogs, because as we have mentioned, he was fucking insane. Eventually Barbot was caught and executed, and Papa Doc kept his head. You know, for Voodoo.