The Follower of the false Emperor are going to die first, rest is minor competition. Orcs will be easiest, get Papa Nurgle some Skin Lotion and the Orcs will think its Gork or/and Mork
their forces will still continue to replish ours as their "loyal" subjects are revealed to the truths of the universe. So by attacking xenos, we're still sort of hurting the Imperium as well!
Think about a meal, you always start of with a starter (Eldar) then move onto the main course (Orkz) and then onto desert (Imperium). That's how Khorne would probably prefer.