From the story forums
At the very least.
Because of his extremely bad judgement and his willingness to appease his people, he put Garrosh in charge of the Horde. This is despite all of his advisors, and even Garrosh himself, objecting to this idea, as no one saw Garrosh as a good leader (Garrosh wished to remain a military commander).
He's the one who told Garrosh how awesome Grom was (despite Garrosh knowing his father had anger issues), without bringing up any of his flaws. After seeing hew good Garrosh can handle an army, he immediately assumes that he will be as good as a political leader. Why? Because the Orcs respected him.
He ignored all the damage Garrosh was inevitably going to cause to both the Alliance and Horde, and went back on his word (Orcs should live in Durotar as punishment for their actions), and hands Garrosh the reigns simply because he put the values of the Orcs above the other Horde races. Hell, in ToW he says what Garrosh is doing to both the A and H isnt his concern because hes in the Earthen Ring now.
Seriously, the only reason the Alliance shouldn't kill him outright is because of his actions in Cata. Many members of the Horde would obviously reject executing him because of all he did for them. If their is any sense of realism in this game, Thrall will be exiled following Garrosh's death.