In the Stonetalon questline, he came to the defense of a tauren druid, punished officers for threatening innocent lives, and was absolutely horrified by Sylvanas' pragmatism. Sure, he was willing to do what he needed to preserve the Horde, but he was beginning to see why Thrall wanted to avert a war.
Come Mists of Pandaria, he doesn't want anything to do with Tauren (or any Horde race, including half of the orcs of the Horde), bombed the neutral settlement of Theramore and is willing to risk the whole of Pandaria/Azeroth to beat the Alliance in a conflict primarily escalated by him. He's not carving out a spot in the world for the Horde, he's remaking the world for his legacy.
He jumped from mopey and pathetic in TBC, to brash and impulsive in Wrath, to brash and honorable in Cata... to cartoonishly evil in MoP. All of his character development just dropped out from under his feet without any real catalyst.
They had the potential to turn Garrosh from facepalm-worthy jackass to a genuinely decent character who represents a coming-of-age perspective over the course of these expansions, whose aspirations of glorious battle could have died off in favor of maturity and wise leadership, but then they just destroyed his motivations for everything for the sake of killing him off.
It's not just Garrosh's character development that's disgusting either, it undermines Thrall's judgment in letting him lead the Horde, turning Thrall from a wise leader vying for peace into a complete moron over the course of one decision (two, if you include Gallywix). It undermines the entirety of the orc race's philosophies for ever listening to Garrosh. It undermines the rest of the Horde's leadership for ever putting trust in Thrall.