The IFCC has something to say about that in both directions. On the one hand, they are themselves a commission of cooperation between competing factions of evil from the lower planes. Evil can work with other evil. On the other hand, in their words, evil isn't one big happy famiily. What they want is destructive, unnecessary conflict, and that may well include conflict on the lower planes.

It's like that xkcd comic about standards. You have fourteen competing factions. You try to resolve things by bringing competing factions together under one banner. Result: you have fifteen competing factions.