Given all the much more interesting characters they killed off over the years, I really don't see the issue with them killing off a guy they pretty much set up for this.
"Petty revenge" is down playing the murder of thousands of innocent people. People instinctively want retribution- do not act like you're above it.
Who are these innocent people? Are you talking about the army of the Alliance at war with the orcs trying to wipe them out? Or maybe you were referring to the military base of Theramore that launched an attack on the Horde in 4 zones and Garrosh bombed after civilians had been evacuated?
I honestly don't get people like this.
People want to know why Blizzard made Garrosh just blatantly, foaming at the mouth evil instead of a bit more nuanced. Well - they'll probably point at stuff like this. This is the reason.
"Look, we tried. We really did. We wanted Garrosh to be one of those people who genuinely thinks he's doing good for his people, but then just gets swept up by his own nationalism. Y'know, a monster in the eyes of his enemies, a tyrant in the eyes of his own people, whose supporters go farther and farther, all in the name of the Horde until they can't even tell when they crossed a line. But he had noble goals at the start. It just didn't work out how he hoped it would.
But, well, people couldn't follow that.
At this point, we're pretty much resorting to screaming, right at the player. GARROSH IS EVIL! HE IS A BAD GUY! HE IS NOT NICE! HE KILLS PUPPIES AND PUNCHES OLD LADIES AND DOESN'T WASH HIS HANDS AFTER HE USES THE BATHROOM. BECAUSE HE IS EVIL!
... We were worried it was a bit over the top - but we're still having trouble getting through to some of the playerbase."