It was garrosh's exposure during fogs of WOW Classic Burning Crusade Gold pandaria to the evil gods called the sha Gods which drove him insane and gave him that hulking, dire troll-like physique. Before that, in the Icecrown Rally, you can see garrosh's smaller stature , and that thrall is larger than him. He also joined up with the iron horde and got stronger thanks to those orcs.

I'm sure he trained very hard with his "real gangsta iron horde orc bros" to get bigger and stronger.

In addition, I do believe that Thrall was reserving his words for the entire fight because in his mind, he believed that there was a problem with him. He remained that way throughout the entire time until he had the conversation with his mom in Korthia.

He was aware that garrosh would have to be killed, but only as an last resort. Sure, he could have gone the warrior way and cut off his head or cleave him with an axe, or brutally hit the head of garrosh or smash his head using the doom hammer but perhaps that would have been too brutal. He chose to use the electric chair option which allowed him be killed by only one bolt of lightning. I'm assuming death was relatively painless to garrosh. A moment later, the lightning strikes and he loses all senses. He dies immediately after the shock.

I'm trying to communicate that the warrior within thrall was not able to kill him. Because that would've been the most brutal and painful death for garrosh. Honor is the responsibility of the warrior. The Shaman is the wild one. The elements are more wild and untamed. They are able to be pushed and pulled, and they will destroy your life almost immediately.

It's possible that thrall lost his warrior skills because of lack of training as he mainly uses his shaman powers since the war ended with archimonde in Warcraft 3. Then perhaps thrall had become weaker as a warrior at that time. Now that he's holding dragora the throne of warriors, he's more like and present day thrall could likely kill the makgora garrosh with an Axe throw "for Azeroth!" Like it was the case with Helya!

Although Thrall killed Garrosh using his shamanistic skills, he still felt guilty about how he handled the circumstance and how buy WOW TBC Gold Garrosh ended up dying due to the decisions of Thrall and all the other factors that led to him losing his powers.