Every Mythos talks of a war in heaven, of opposing factions of gods, often described as good vs evil.

Of course, it's metaphorical and just a representation of humans finding some of their less moral counterparts being.. well, immoral, which is a universal traits. Some people are just bad people. Morality may be subjective, but through history, we've had a pretty consistent baseline universal morality that every functioning society seems to have agreed on. When this breaks down, usually empires fall.

The problem is when in societies, the bad people start outnumbering the good people.