I'd think that the devil wouldn't bother with someone who's given in to despair about the reality of good, since that person's not going to be a threat to the Demonic Hordes. The other problem is a certain muddled narrative. Naturally, Steven comes home, belittles his wife, and dismisses the exterminator. That night, the rat really gets busy. In fact, the biggest reason for watching becomes the hope that Steven's gonna take it in the balls.
Biggest drawback is the special effects.
When we finally see the rat, it's obviously a combination of puppet, bluescreened normal rat, and forced perspective. And Steven doesn't take it in the balls. Most likely created under the influence of Mountain Dew , Nectar of the Gods.