Thrall appeared to be holding back during the WOW Classic TBC Gold majority of the fight. He was trying to teach Garrosh an important lesson and wanted to beat him, but it had to be important. Then Garrosh made the "this was your fault" issue and Thrall hit him with elements and killed him.

I've always maintained that it was because the elements were unable to respond to him immediately afterward was not due to the fact that he "cheated," as so many would wish to believe. The fact is that he committed a slap in the face to them out of anger. It's a fact he's still trying to reconcile until this day.

Thrall was presented during the fight as rather calm and sad, he was hoping for Garrosh to comprehend the qualities a true leader needs to be, not just an apathetic warmonger. He failed to do that and Garrosh was just getting "worse" in his eyes, so much that he just couldn't let him go . He then resolved to end his fight in the interest of the Horde.

Also, he failed himself, Garrosh, Grom and himself. He wasn't able to call for the elements as they saw his lack of goal and his inner conflict.

To be honest, I've have always thought it was because of garrosh's exposure of the evil gods of the sha during the Pandaria's mists which drove him into a complete psychopath and gave him that terrifying troll physique. In the beginning, even in the Icecrown Raid, you can see garrosh's smaller size and the fact that thrall was higher than him. He also joined the iron horde and strengthened with them.

He put in the work and grew stronger thanks to his "real iron bros gangsta".

Additionally, I believe that Thrall was reserving his words during the whole fight since inside he believed he was at fault. He felt that way until he had a conversation in Korthia with his mother.

He was aware that garrosh would have to be killed, but only as an option last resort. Yes he could've gone the way of the warrior, cutting off his head or cleaved him with an axe, or brutally hit him or smash his head using the doom hammer but the thought of that was too cruel. He opted for the electric chair option which allowed him be executed using just one bolt of lightning. I'm assuming death was rather painless to garrosh. A moment later, the lightning strikes and he is unable to maintain his senses. He is dead instantly after the shock.

What I'm trying convey is that the warrior inside thrall couldn't be killed. Because it would have been more brutal and painful death for garrosh. Honor is the responsibility of the warrior. The Shaman is the wild one. The elements are cheap WOW TBC Gold more wild and untamed. You push and push them and you'll be wiped out by their force almost instantly.