Do you personally think that if he had conquered Ashenvale, Garrosh as Warchief have burned down Teldrassil? Like Sylvanas did?
I think, even to this end - Garrosh had some definite sense of honor and dignity. He didn't hurt Anduin, for example, after he found out Anduin had spared his life during his trial - and he could have easily maimed him. He might have been willing to kill others - but if the Iron Horde won, I'm not sure he would really try to exterminate the other races, unless he was seriously corrupted somehow.
I think he COULD have burned Teldrassil - in a very fit of passion and rage, and perhaps if he felt it was a choice between his people's continued survival and existence and that of his enemies. But I don't think he'd be happy about it. Or celebrate it. Perhaps he would act happy at first - but eventually, not immediately, he would felt definite unease and guilt at his actions, perhaps months or years after Teldrassil was destroyed, and he might hide it, but it would slowly and surely gnaw away at him from the inside out. He acts tough and brutal - but he is not without conscience. We know that some night elf civilians were slaughtered - the short stories tell us that, and some NPCs like Leyara lost their family members to the Ashenvale invasion - but I don't think Garrosh necessarily targeted them specifically, they were most likely "collateral damage", perhaps slain in the bloodlust of the initial attacks.
But the upheaval of the natural order caused by the Cataclysm—and an influx of orcs— had changed everything. The orcs had slaughtered Sentinels and civilians alike. Cordressa had not seen it with her own eyes, but the orcs had reportedly hunted down those who tried to flee, letting their bodies rot in the road as a brutal warning to anyone who would try to retake Silverwind Refuge.
And perhaps people forget - the bombing of Thal'darah Grove, which killed a bunch of innocent children, was not sanctioned by Garrosh. He actually killed the orc who was responsible. "Honor...no matter how dire the battle, never forsake it." He kidnapped magnateur children - but he did not torture them, and though they were killed anyway in Kalimdor, we're not sure whether it was Garrosh's mistake - perhaps he was not aware of the dangers, or perhaps he believed they could defend themselves.
Do you remember why Cairne died? Because he thought Garrosh had supposedly attacked a group of night elf and tauren druids in Ashenvale - but the reality was, he hadn't - most of the Horde thought he did. But he refused to do that. That was not his way of fighting, nor his way of honor.
Indeed, he was pleased at Theramore - but most of the military deaths were overwhelmingly military, and we are not sure whether he was aware most of the civilians were evacuated. I personally think he was an incredibly complicated - and deeply wounded - character who had considerable development, both positive and not that positive, and lots of wasted potential to his end.