Vol'jin has a pretty big potential problem here with Garrosh. Whether you like his tactics or not Garrosh had a pretty large following among the orcs. Leaving Garrosh alive gives this faction hope of his return. Killing him provides them a martyr. Which is more dangerous and how does Vol'jin deal with this? How much say will he have? If the visions of time video is true, we already see Garrosh in a magical cell somewhere on Pandaria. Not sure if this is his temporary cell during the trial or something permanent. As much as Arthas was the Alliance's problem, I see Garrosh as ultimately the problem of the Horde. They should have final say on his fate.