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.
There are several factions in the Eberron setting which tend to have very long term plans relating to attempts to fulfil various prophecies. The suggestion is that agents of those factions will often help, or even become patrons to, the PCs, because saving the orphanage (for example) puts in motion a series of events that assist the fiends in achieving their ultimate goals. If the PCs realize the nature of their benefactor, do they continue to do the genuinely good deeds their benefactor is helping them to do, when they have to suspect that the end game is something much more sinister?
The other side of the coin is the lawful good but ruthless benefactor who asks the PCs to do increasingly disturbing things, trying to foil the plans of the evil factions - eg. burn the orphanage and put the orphans to the sword. Moreover, both good and evil factions can be mistaken or misguided AND detect alignment magic can be unreliable in-setting, so if the PCs want to know what is really going on, they have to do some investigating on their own. Gygax has sages, and Eberron has Lovecraftian university professors to fill that role.