We can see hints this even earlier, when Guts and Puck meet up with wagon girl and her father. When the girl gets possessed and kills her father, Guts has a look of horror on his face as he flashes back to how innocent she was before and freezes enough to get stabbed. When he snaps out of it and kills her, he's trying not to vomit as he fights the demons around him, and finally reverts to bestial flailing to survive. In the end, he's leaning against his sword glancing at her, and Puck attempts to console him which he quickly rebukes with his "people who are weak die." The demons of the forest taunt him, which he responds by firing his arm cannon in an act of defiance, while at the same time having a face of childlike terror and desperation/hysteria in his eyes. Puck doesn't even criticize him, he just looks around sadly at the kind of life guts has to live.
It's a fantastic sequence, even in the earlier, "rougher" chapters.