High-rank demons take things to the next level by invading planes and using other races as their cattle. Their needs grow with the increase in strength and so does their appetite. Other races are better than demonic beasts because they have complete souls, which means more soul essence for each kill.
The difference in value between killing a demon and other races of the same power reduces to 10 times instead of 100 for demonic beasts. Killing other races is more than 10 times better than killing demonic beasts.
No matter the level of the demonic hierarchy, competition and conflict are a staple. It could occur because of resources, a clash of sins or ideas, or simply distaste. Demons of greed like to steal and demons of wrath like to get angry. Those two examples are more than enough to cause conflict in any society.