Thrall displayed a calm, sad WOW TBC Gold expression throughout the fight. He desired Garrosh understand the difference between an authentic leader and a warmonger. He failed to do that and Garrosh was getting "worse" in his eyes. So much that he was unable to let him go . He then decided to end him for the sake of the Horde.
Also, he failed, he failed Garrosh along with himself and Grom. He was unable to call for the elements as they recognized his lack of cheap WOW Classic TBC Gold motivation and internal conflict.