Death Knights have been traitors since day one. I can't understand why Tirion didn't just kill those mongrels on the spot at the Light's Hope Chapel. Well, okay. Tirion was a follower of the Light which means that he has to do good deeds but Grommash shouldn't have allowed DKs to join the Horde. Same goes with Varian.
Death Knights touch dead bodies. They steal humans. They resurrected Nazgrim even though Vol'jin didn't allow it.
Hopefully Khadgar will shoot down Acherus in 7.2 or something.
By the way, death knights attacked an innocent holy priest Revil Kost and almost kill him just to get some information during the artifact questline. Why hasn't Anduin condemned that?