I'm going to put this out there for posterity's sake if nothing else: I believe Malkorok (Garrosh's de factor majordomo as of the latest patches and lore) is actually the Dreadlord Detheroc finally making an appearance. I don't believe Garrosh himself is corrupted by Fel power or even explicitly allied with the Legion, either - I believe Malkorok is just Detheroc doing what Dreadlords do best, pitting nations against another nations and then dividing those nations even further inward and apart until everyone is left alone and afraid with their world ripe for the Legion to invade. Malkorok's agenda of supporting Garrosh (already a divisive figure), squashing dissent, and almost ensuring rebellion within the Horde seems to fit the Dreadlord modus operandi to a "T."