Dealing With the Devil
Allow me to put my theology cap on. My demonology isn’t all that refined as that was not actually covered with anything more than a passing reference in my course of study, but I do know a few things. For one I know demons masquerade as angels of light; I also know Satan’s goal is to gain worshippers of his own, not necessarily to destroy, to create chaos, to corrupt into gross acts of sin against God (though I cannot deny that he might approve of such tactics; see Job). Is it so hard to believe that a person would cooperate with an angelic figure who merely wanted their worship? Then is it so hard to imagine that a person who might have sold his soul to the devil might not be a dark and sinister man?
Go forth and write. I know of few characters who made a pact like this, but it seems far more likely than the usual means of making a contract with demons and the devil. And no, I have no ideas how one might actually go about contacting a demon in order to make this contract in the first place, I’m assuming it will happen by chance when least expected, and it will seem pleasant and beneficial.