Not that I'd ever want to defend this show, but since Maou is actually nothing like a king of demons in the first place (or actually second place, since in the first place of the other world he did seem like one), it makes more sense. After all, Hero should be... You know, a hero. So, if a hero noticed the maou is actually acting like a lowly human toiling his days away at an honest job, why would the hero anymore want to slay the maou?

But other than that weak, limping half-explanation, there's nothing in the plot supporting it. It just happened to serve the (largely missing) comedy.