Why are we arguing abut this? Yes, it's Thrall's fault, he accepts it's his fault and wants to fix the mistake he made (he says this confronting Garrosh). Thrall didn't think he primed a weapon that would be aimed at the Alliance and blow up in the Horde's face. Garrosh's tutelage under his guidance as well as Saurfang, Eitrigg, Cairne and Vol'jin was supposed to temper him, but it didn't.
Thrall made the mistake of putting him in charge, he did not do it maliciously, twirling his mustache while Garrosh destroys the Horde and incurs the wrath of the Alliance. I suppose the parallel between the Titans and Mogu story from 5.2 was lost? Thrall is the Titan, Hellscream is Lei Shen, his loyal orcs are the Mogu.