A demon that committed pure evil acts (minus the sexual ones, even he knows those are a no go) and created pure destruction grew bored and decided to try being nice to see why humans liked it so much...and ends up addicted to the warm fuzzy feeling of good.
He goes on to become a cleric in secret, meeting the god that governs goodness and vows to teach the rest of hell the glory of kindness..mostly by bashing their heads in, until they all agree.