3's a Lonely Crowd
The one thing Khorne really doesn't want to see is the Old World restricting his capability to fight. At that point, because Slaanesh already has his Field of Ecstasy cards and Nurgle potentially has Rain of Pus (increase defense), Khorne's pretty much resorting to getting points. He's going to help corrupt the pacified regions to ruination so he can get back to his main game. There's no free Tzeentch food for him.