I was out leveling my monk on the Wandering Isles (Zhenyi on WRA, so people can file that information away for later use to tell me how much I suck at healing when I try to talk about endgame PvE ) and people were discussing the Warchief a decent number of people love to hate (who has cultivated his own notable fan base through Cataclysm). Suddenly, I started going over his character progression and something clicked.
Garrosh wasn't some froth-mouthed madman in Wrath and Cata--he was the same scared orc who couldn't handle the stresses of leadership that he was in BC. After Thrall told him the other half of Grom's tale, the heroism as well as the villainy, Garrosh began trying to overcome his own shortcomings to make his father proud, and overcompensated in his behavior. He wanted to be a 'real' orc like his father was, rather than let the Hellscream legacy die with a simpering weakling cowering by a bonfire. People buy into the act, and he becomes the hero he wants to be--he's given command of the Horde's forces in Northrend without having been tested as a leader or an officer at any point before. He was thrust into the role because of who his father was and because Thrall wanted to do Grom's legacy right.
That's when he started to crack. He made numerous tactical blunders, and rather than admit he screwed up (which he felt would cause him to lose face among his troops), he instead became increasingly-aggressive and kept shifting the blame over to the Alliance. In doing so, he hoped to disguise his shortcomings by directing his frustration and his troops' ire elsewhere. As the War against the Lich King carried on, he became increasingly hostile and brutal, which many of his commanders took as the behavior they should espouse as well--one instance, the Broken Front, left Garrosh horrified. It was what alcoholics refer to as a moment of clarity, one that lasted all too short and went ignored.
Due to his success in Northrend, Thrall felt he was the most obvious (surviving) choice for Warchief. This promotion caught Garrosh completely off-guard and the role was largely dumped in his lap with no training or preparation on Thrall's end to prepare Garrosh for a life of global politics as well as military command over more than the Warsong. Completely out of his element, Garrosh reverted to his brutal and aggressive leadership stance, which, while it worked well with orcs who were desperate to recapture the 'glory days' he so often spoke of (likely only referring to his father and Doomhammer, rather than the truth behind said glory days), thoroughly alienated the more moderate Horde races. His refusal to back down and accept the possibility that his people might have done something wrong caused multiple situations to spiral hopelessly out of control, because that was the only way he knew from Northrend, and had become so set in that defense against a role he had no training or credentials for that he was unwilling to consider other options.
As his tenure as Warchief went on, I think he broke under the pressure of long-term leadership. His advisors, people he respected and admired on first joining the Horde, openly hated him and did their best to undermine him when they thought they could get away with it, the other leaders refused to follow him when he tried to remind them that the Warchief's word was law in the Horde, and the war with the Alliance was proving bloody and costly. I think after Deathwing's fall he simply snapped. Suddenly, he realized why Krom'gar had used the bomb--to minimize loss of life on his side and ensure there would be no survivors to seek vengeance. He understood that if the Horde leaders didn't respect him, fear would work just as well in keeping them obedient to their Warchief. He understood that if the Alliance wouldn't let the Horde have Kalimdor, there was no reason for the Horde to let the Alliance have the Eastern Kingdoms--indeed, that if this infighting continued, Azeroth would suffer, so to his broken psyche, the best route was to wipe out the Alliance and allow the Horde to defend Azeroth without the distraction of the Alliance to deal with. And from there, he only gets worse, losing himself further in his madness as time goes on and more people abandon him rather than stand up and get him to back down, until those heroes of the Horde he once respected and admired turn on him in a bloody civil war.
Like I said, maybe this is just me overthinking things, but if they really did plan this from the start, to have Garrosh as a tragic figure broken by the leadership positions he was never capable of handling mentally, I'd have to give Blizzard a well-played of the highest degree.