To be honest, I've have always believed that it was due to WOW TBC Gold garrosh's exposures to the gods of evil during the pandaria's mists that made him completely mad and gave him the terrifying troll physique. Before that, in the Icecrown Raid, you can clearly see the garrosh's diminutive size and the fact that thrall was more powerful than garrosh. He also joined the iron hoard and was strengthened by them.

I'm sure he trained very hard with his "real Iron gangsta gangsta orc bros" as he grew larger and more powerful.

I believe that thrall was seeking to cover up the fight because he believed he was at fault. He was still feeling that way until he has the conversation with his mother in Korthia.

It was only as a last resort that he realized he needed to kill garrosh. Yes he could've gone the warrior way and cut off his head, or cleave him using an axe or brutally punch him or smash his head using the doom hammer but maybe that felt too inhuman. He ultimately decided to opt for the electric chair option of execution in a single swift strike of lightning. I'm assuming death was fairly painless for garrosh. He is struck by lightning and immediately goes blind.

What I'm trying say is that the warlord in his thrall could not kill him. It would have been more brutal and painful death for garrosh. The warrior is a hero. The shaman is the wild one. The elements are more wild and untamed. You push and push them and you'll be destroyed by their power within a few seconds.

It's also possible that thrall has lost his warrior like skills due to a lack of practice as he mostly just uses his shaman powers since the war's end by using archimonde Warcraft 3. Perhaps thrall isn't as skilled in the field of battle since this time. Now that he's holding dragora his strength is greater and he could possibly kill the makgora day garrosh with an axe toss "for Azeroth!" Like He did to Helya!

Even though Thrall killed Garrosh with his shamanistic powers he still felt guilty about the way he handled the situation and how Garrosh cheap Burning Crusade Classic Gold came out because of the choices made by Thrall and the events that led to him losing his power.