The bad guy campaign picks up right after the Downer Ending of the human campaign. Turns out that demonspawns are The Virus. The Hero and his army were assimilated, but still retain their humanity. And so they continue fighting For Great Justice, despite being demonspawns.
Parodied: All the selectable factions are complete goody-goodies; the non-selectable factions include those that Poke the Poodle.
The game is advertised to let you play as the bad guys as a selling point. Once you actually get the game, you don't actually get to play as anyone except the good guys. DLC is later released to let you play a villain's campaign.
There are multiple good and evil factions. Some good factions have campaigns, but others don't. Some bad factions have campaigns, but others don't.
Averted: There's a campaign for the bad guys.
The bad guys are really bad, and the execs are afraid that letting players control their slaughter would result in really bad PR for their series, so they don't allow it.
Alternately: the evil faction is structured too differently from the good factions to implement a campaign without fundamentally altering their game play.
Reconstructed: When you find that out, however, you join them and from there on out there's no campaign for the previously thought good-guys anymore, as you try to save the world from the Evil Overlord you helped put on the throne.