Suppose you served demonic creatures from a plane of existence outside of our own. These creatures for whatever reason absolutely delight in human suffering and basically nothing else. But each of them likes different kinds of suffering. Depending on which one(s) you served, you'd behave in different or even contradictory ways that might not make sense to a casual observer. Because the intent behind the actions you take is an alien, inhuman one.

That's why the enemy isn't united. Because their masters have different competing interests, even if they share the same category of being evil.