@Doomer You wouldn't get the impression from cheap fiction from mediocre authors, but in reality a villain's worst enemy is not any hero, it's another villain. They are villains because they are nasty and wicked. They don't have any friends in the first place. They might have allies when interests are aligned, but they would always need to fear backstabbing. They would have underlings, but the most capable of those would always be plotting a nice little murder and usurpation, the weaker would only serve through fear or because they are too stupid to have ambitions of their own.
If you think about criminal gangs and organisations in our world, most of their casualties come from fighting other crooks. Similarly nothing would be more natural for a demon than to slay another demon to steal its position and power.