I'm going to argue that that was a change in his personality that was made after Blizzard decided to write him off as a bad guy. Again, top-down decision.
Through WotLK we saw Garrosh being guided by Saurfang and Thrall to act with "honor" and for-thought. In Cataclysm he was still brash and aggressive but there was still plenty of indications that he learned those lessons of honor. I mean, He was still aggressive about Horde expansion, but he didn't condone the killing of children or the use of the plague. He was really being built up into a legitimate Thrall replacement, one that was less diplomatic and more of the "war" in warchief.
Then MoP hits, and it feels like all that get's suddenly gets tossed out. Garrosh is suddenly "win whatever the cost" and "only the strongest have any value;" whatever lessons Saurfang had taught him are 100% forgotten, just like that? okay.