Wow, talk about messed up. The first two chapters really got me hooked. The concept is cool and explains why lots of people have psychotic breaks for no reason. That being said, it kind of takes away all responsibility from humans of every evil action they've done. No one could be held accountable for what they'd done, least of all by God, because the devil literally made them them do it. They couldn't fight it and didn't have a choice. Doesn't that take away their free will too, that the demon claims they do have? Still an awesome idea, it just doesn't quite fit with just morality, or religious morality, which this story seems to be based around.
Read the story now
The beginning and ending are very well done and thought out, but chapters five and six are mainly just torture porn, not really scary so much as just gross. The ending would have been truly terrifying if the chapters leading up to it had been just a tad more subtle and geared toward the demon's methods of breaking people down, or by making them do little things at first, tiny sins everyone does.before moving on to the heavy stuff. It's hard to convince somebody they could become a serial killer in moments, but a cheating spouse, a thief, or just generally mean person. That would scare me more, the idea of a demon slowly changing me from the inside out with the little sins that lead to bigger ones later. Just an idea.
I think you should've called it "The Devil Outside", or "The Devil Above", since the demon is looking down on the humans from the air, at least he seemed to be, until later when he was just walking around with them. Plus it would subvert the trope.