He did what he could, and that was to stop them in Zul'gurub and Zul'Aman. He had no way to stop Zul, nor did he know about Pandaria or their plans or the mogu and the Thunder King, besides he only just recovered, he was still recovering by the time Lor'themar assaulted the Isle of Thunder. Oh and joining the Horde? Cut the crap, that was never an option - never.