Wasn't that just for show? If Ainz can truly create death knights from a head, he wouldn't be making skeletons. Or maybe death knights require far more mana but he pretended it was like nothing to freak the humans out.

About the Emperor's decision, what he really should be doing is investigating what Nazarick wants instead of instantly plotting against them. That's literally the last thing you should do to an omnipotent ally, even more so because of how delicate that status is. His fear of Ainz wiping out humans is not consistent with the honorable treatment he and his people received at the tomb. Resolving that inconsistency through communication and investigation is the first thing to do, not randomly suicide your entire race against a god.