Not religious myself, but I always thought in the grand scheme of things it made more sense that the devil was a jail keeper rather than an antagonistic being.

In the Old Testament Satan was pretty much on Gods side. His job was just to tempt and judge humans, which meant he was kind of a asshole most of the time.

In most modern christian lore, Satan isn't really in charge of hell. He's less a king rulling over a domain, but more a mobster causing trouble in a lawless town.

Basically, what im saying is that you are somewhat correct. Satan did use to be on Gods side, but that more or less changed the more the lore developed and they needed a clear antagonist. I don't think he's ever been in charge of hell though. Even Dante's Inferno has him as another prisoner rather than the head honcho.