Time and again an adventure will introduce an NPC, monster, or some such that makes an alliance with the party, hires them, or otherwise does something that does not include "Stabbing the party right now." Also, it has now become a trope that any alliance the characters make with an evil group or character will end with the party getting back-stabbed by said NPC/creatures. Evil, it seems, is dumb.
This is boring. It limits the emergent play possibilities that make RPG's fun. This is not to say that the evil group can't have their own goals and motivations, but consider the play possibilities of NOT backstabbing the party. The roleplaying possibilities are substantially greater when the characters get to interact with evil NPC's and creatures. What's the line of what the party will accept? How obvious can be the creatures evil actions be before the characters have had enough of them? What if the evil necromancer offers resurrects to the party for free? How much will the characters then put up with? What if the friendly ogre is wearing The Ruby Crown Of Ultimate Power. Who's the bad guy now when the party starts stabbing?
This is to say that everyone the party meets from Team Evil needs to be friendly to them. And, yes, Evil can advantage of weakness when they see it. That's all fine and fair. But mix it up a bit; they don't all need to be psychotic killers.