He is a hero. One of the lead heroes. He is personally responsible for leading his faction in their slaughter across the world into demon territory. How many similarly distressed enemies has he left behind crying over their dead friends?
As for his immaturity, he lost it when he witnessed Kai kill the saint, the leader of his faction, and yet later did not entreat him as an enemy. He clearly demonstrated the ability to comprehend and maneuver through morally complex situations. That was months ago in story time.
Yet the simple idea of one comrade's death not negating their overall success outrages him?
As the lead fighter and one of those with the highest body count in a war he chose to participate in, he has lost ANY right to be outraged. And I am fully justified in wanting to slap some sense into him through the screen.