"God Is Evil" is a trope in which God - who rules over everything in monotheistic religion - exists, but is evil. He demands absolute fidelity and threatens eternal punishment for those who fail to comply, whether out of twisted idealism or for sadistic pleasure. Despite this, Jesus is normally portrayed as a nice guy. He might be duped by his father, or killed because he didn't stick to the plan, but he is still good. The Devil might be good in such settings;a rebel against an oppressive monster of a God who happened to have bad PR. Then again he might be every bit as evil as God says he is, making the whole matter a choice between two evils.
For those without a Judeo-Christian worldview this can be even easier to accomodate - the Greco-Roman pantheon were worshipped despite being (at best) amoral and unpredictable and the Aztec deities were mostly utter horrors. The original Cthulu Mythos contained very few immortals who weren't malevolent. The idea of divine powers who are both interested in humanity and benevolent is far from universal - making them into a monotheistic or monist entity shouldn't challenge much.
You don't even need an existing religion for this to work. All that must be noted is that the divinity in charge of the cosmos is evil. If there is an evil deity opposed by a pantheon of good or neutral deities, then that is God of Evil.
Game and Story Use
- This is perfect for Rage against the Heavens plotlines, in which the PCs are mortals who choose to rebel against the divine oppressor.
- This was - pretty much - the basis of Catharism, should you need guidance on how to establish a belief system along these lines. As the Cathars saw things, the Demiurge was evil and created the material world, but wasn't the true god. He was trapping souls created by the true god in his creation.
- For Catharism to actually match this trope, the divinity ruling over creation would have to be evil. So it's not exactly the same as this trope, but just close enough that the it should still be easy to mine for lots of ideas.