Not so sure AU Gul'dan is much of a Legion loyalist either. He frequently talks about power granted to him and Archimonde apparently thought Gul'dan may abandon them as he grabbed him and sent him to our world with that bitter "we made a pact" phrase. I think he'll do whatever the Legion asks as long as they grant him more abilities. Should there come a point where he has the opportunity to slay them in order to gain even an ounce of more power, I think he just might screw them all over.