...Belmarniss did tell her to try to consider other viewpoints, and to remember that she might be wrong about thing. And Tanya is trying, but -
"The people I harmed in war - directly - are mostly people I killed. You're saying I should... consider some of them might have gone to hell, and that is too great a price to make them pay whatever their crimes against me?"
What would have happened if Germania had somehow (ridiculously) surrendered instead of fighting the war? If it had surrendered to Legadonia, in the very beginning, or at least opened peace talks instead of counterattacking? And all the Germanians, military and politicians and the ruling family and the people had agreed to this?
The Francois would attack them, of course. They did it as soon as they saw an opening, and that opening was Germania winning against Legadonia, just because they didn't win quickly enough.
So what if they surrendered to the Francois too? Well, presumably the commies would be next, racing to bite off the eastern provinces while Germania was busy being conquered in the west.
If they just - disbanded the army, declared Germania an open country, fuck, disband the government while you're at it. Just have everyone peacefully till their land. The Francois and the Legadonians would negotiate some agreement to divide the western half of the country between them, and the commies would take as much as they could without fighting the Francois. Some number of Germanians would die, certainly; armies are not gentle when they conquer a hated enemy, even if there's no army to oppose them. Between a quarter and half of the population would be enslaved, all landowners and nobles and priests and educated people in the East executed or sent to labor camps. In the West, too, the industry would be plundered. And then there would be a temporary peace, while the great powers digested their gains and tried to figure out the best way to conscript the miraculously pacifist Germanians into their armies for the next great war to be fought on formerly Germanian lands.
It's possible to argue - to imagine - that fewer Germanians would die, in the end, than have died fighting the war that Tanya has lived through. If you optimize for the metric of fewest deaths, even without taking the other nations into account, and if you don't care about the quality of human lives in comparison, then that is a rational conclusion to make.
Of course it's not stable. The next generation will be reeducated, terrorized if necessary, and conscripted into armies that are no longer Germanian. You need to die to achieve pacifism on the scale of a nation, and so do your children. But your only concern is for the fate of your soul, so that is not a problem. You're a model Christian, the kind that takes sermons not just seriously but literally (and also the bishop hasn't been paid to sermonize for the war). You're not even optimizing for the fewest deaths, or the least harm done by people to other people. You're just optimizing for nobody going to hell.
This is a very simple model, with very simple rules. Well, no, actually the rules are unknown and probably complex, because Tanya still doesn't know what Pharasma wants in any specificity. But everyone rational wants to avoid hell. And if it's the rational people who manage to avoid it, well, that's everyone else's problem, isn't it?
The only price is admitting you never cared about anyone or anything else except personal survival. And you knew that all along, didn't you.