I'm aware that the cutscene was crafted this way by the writers since it's a way of ensuring. Thrall had to be defeated before he could get his one-two punch. This was so that we, the viewers, could enjoy the satisfaction.
It always made me feel guilty that it came out as it did. Hopefully this didn't come across as a ranting rant of a fanatic. It's how I view it.
Thrall seemed to be taking a back seat throughout the fight. He was trying to teach Garrosh the lesson and was determined to take down Garrosh but it had to be crucial. Then Garrosh did the "this was your fault” thing and Thrall attacked him with the elements and killed him.