It is not easy to live with a landmine in your own brain, to know that there is an area of thought that any step in can spiral you into a hopeless, useless state when you are needed. The way to deal with that is... well, it's to get therapy, but in the grim darkness of any time period before like thirty years ago, the way to deal with that is to rope off areas inside your head that you absolutely do not go into at any time for any reason, and do your best to live outside of that boundary. When Thorgrim inherited the throne and the crown almost two centuries ago he forced himself into that miserable mindspace to confront the extinction of his race and the inevitable failure of every hope and dream he'd ever had, not just once, but however many times it took to break the stubbornness of the Dwarviest Dwarf that the entire Dwarven race could find. The Thorgrim that we knew - the Thorgrim that canon knew - was a Thorgrim that had, after however many years it took of facing the slings and arrows of outrageous fortune to break him, knew that the Dwarves had lost, and had somehow managed to find the strength to not find solace in madness or death and instead make sure that everyone who had put Dwarves on the road to extinction would know as much regret as it was possible for the Dwarves to teach them with however many years they had left. And one of the ways he had managed that was to prevent himself from stepping inside the parts of his own head that will make him useless. He had stopped looking for hope because he had learned over decades of torment that hope is what tells you that this time you can step on the landmine and it won't blow your fucking leg off. When Belegar took Karak Eight Peaks, Thorgrim did not respond to this with hope, because he had learned lifetimes ago not to.
Canonical Thorgrim, and the Thorgrim that Belegar had clashed with, was not a villain. He was someone that had inherited an extinction and had made a terrible peace with that fact. When he told Belegar to die well, he was not saying 'fuck you'. He was doing what he felt to be an act of brutal kindness to his brother king: outright telling him what the best hope for any Dwarf alive in this cursed time could be. To die well. To have an argument to present to his ancestors that yes, he had failed, but at least he had made the victory of the enemies of the Dwarves turn to ashes in their mouths. Die well, Belegar, because the only alternative is to die poorly. There is no destiny of the Karaz Ankor that ends in anything but death, of one kind or another. I will not burden you with more Dwarven lives for you to explain the loss of to the ancestors you are about to meet - that is not your burden to bear, but mine.
As it turns out, canonical Thorgrim was right. The Dwarves canonically died in a series of ugly and ignoble encounters that made a mockery of everything that they ever were. To carve vengeance out of everyone in carving distance with the years that the Dwarves had left before that happened was, even with the benefit of hindsight, the least bad option available to the Karaz Ankor. That Thorgrim is wrong in this quest is not because he is an idiot, it is because the vote and the dice gave me enough justification to divert them from that fate. Thorgrim did not take you or me into account because he couldn't.
Thorgrim is not wrong for needing a literal message from his literal Gods to pop up on his HUD to realize that the rules of his universe changed 313 threadmarks ago.