yes he is corrupted, but the thing is he drank the demon's blood(metaphor) willingly and with full knowledge of what it does.
this is less taken over and more taking its power for his own, which is rather remarkable to be honest.
i mean, hes controlling the sha through their own energies.
his first actually respectable act, at least in my eyes.
does making him come off as if blizz is watching too much transformers prime and thinking dark energon megatron(aka robot lich king, no really he commands an army of undead transformers raised from all the dead on cybertron) would make a good end boss.