(Thanks so much for the awesome debate! I hope you know none of this goes with anger or malice; I find the subject fascinating.)

Actually, I think the Hitler comparison is quite apt indeed. A war hero who takes the reins of a discouraged and downtrodden state at a critical time in its history; who uses his status and promises of power and glory and assurances of racial purity and superiority to rally his people to war; who runs a cabal of brutal yes-men who hunt down, intimidate/imprison/murder anyone who suggests that what he's doing might be wrong; who is so blind to the costs of the war he wants and so absolutely sure of himself that he overreaches and begins running his war machine into the ground because he won't listen to the advise of better strategists?

He's not just LIKE Hitler, Garrosh Hellscream is practically an allegory for Hitler.

Now, there is a point that I will absolutely grant you. We don't see the lead up to the big confrontation that Vol'jin shows us in the troll starting area. We don't know what they were discussing before they started in with the death threats to each other. I think a clue lies in what Vol' jin says just before the part you quoted:

"You're no Warchief of mine. Ya've not earned my respect, and I'll not see the Horde destroyed by ya thirst for war." (emphasis mine).

Now, in complete fairness, this is conjecture on my part, and so might not be fair to introduce to this discussion. But for all you accuse Vol'jin of not trying to council him--I believe that he DID, many times, off camera (so to speak), and that this exchange marks the end of the last time he bothered to try. His increasing frustration is apparent; and given how we saw Garrosh react to ANY suggestions from ANYONE, in game and in books (Baine, Ettrig, Vol'jin, etc), I think it becomes pretty clear that at this point, Vol'jin was fed up with Garrosh ignoring council, especially after Thrall's specific request to help him. What Vol'jin shows us in the vision is the moment that he becomes convinced that there is no way to redeem the Warchief--that his choice is either follow Garrosh and watch/participate in the Horde's destruction, or turn his back on the Warchief for the sake of preserving both his own people, and the Horde.

Remember that even after all that, after speaking with Thrall, Vol'jin remains loyal to the Horde, and agrees to try yet again. Of course by that time nothing was going to change the course of things--but it shows that Vol'jin is not some unthinking traitor who didn't give Garrosh a fair chance. He tried, and ultimately (correctly) understood that his loyalty to his people and the Horde were incompatible with loyalty to Garrosh. And so he rebelled.