
This should be an easy one for Christians and Muslims who both believe in a Hell where Satan lives and gets our souls to torture for eternity in unimaginably horribly ways.
The story goes that God, ever the narcissist, created angels so they could worship and adore him, but, so that they could do so in all sincerity and not just as pre-programmed automatons, he gave them free will too.
For some reason, Satan wasn't convinced by his creator's majesty and perfection and, like an up and coming young ape, or a young would-be tribal leader of Bronze Age hunter-gatherers, he challenged the old silver-backed alpha male, lost, and so was chucked out of Heaven for eternity. It's not clear why he wasn't convinced by a perfect god or why this god created him in the first place when, being omniscient, he knew all along what was going to happen. Nor is it at all clear why this god can't forgive Satan and so end evil, but let's run with the story and not get bogged down too much by its absurdities.