I think they wrote Garrosh into a "gray area" because they were undecided on where he should go, in terms of the storyline. They were testing the waters of the playerbase as well, to see if we were hungry for a full-on Horde versus Alliance story -- something that played heavily into Cataclysm, and MoP. The reaction from the players was not as vehement as they thought. People didn't take to Garrosh and see him as the leader they needed and wanted. There wasn't much fist-pumping and hooting for Horde dominance. The player feedback was more anti-war than pro-war. That threw Garrosh's characterization into a tailspin and ultimately, come MoP, the writers decided to make him an outright madman, and villain.
My opinion remains that they knew his direction from the beginning, probably the only doubt has been when someone found him "likeable", a thing that surprised Blizzard, since they said that didn't expect that, and builded Garrosh as a dislikeable person. If their goal was to put Garrosh in a constant bad light (like they did all the time in WotLK) their direction was clear. In Cataclysm they just showed that Garrosh in the end wasn't an utter monster and had some boundaries that he probably learned by Saurfang, even if not respected so fervently and with a lot of personal (and dangerous) views about them. But all the novels, even Wolfheart that have its events happening before Cataclysm, shows how Garrosh's path was well intentioned but inevitably tending to something ever and ever worse.
I'm unable to see at Garrosh as a long-standing character, but much more as an excuse for bring some mess (and development) for refuel the story, since with characters like Kael'thas, Illidan, Arthas and Deathwing dead (the "hooks" from the RTS) they needed something ready to carry on the story further, and the concept of Garrosh Hellscream they had in TBC was for that.
Garrosh has never been 'good', but he was still honourable. Even in Wolfheart, he valued honor in battle, and he even valued the Night Elves as honourable opponents. So yes, you're right that he's usually portrayed as a warmonger, but he was not a monster who resorted to underhanded tactics like murder and hurting innocents. This change came with MoP, and most likely because he was decided to become the big bad of the expansion.