The scene: A band of fighters is moving from one town to another, through not-zombie territory. They're attacked, outnumbered, but in the end they win. One member, the son in a father-son fighting pair, is mortally wounded. They can't stay with him until he dies, can't carry him with them.
They're armed with knives and guns, and used both in the battle (so no worry about the gunshot attracting unwanted attention). They've killed not-zombies by "stabbing them in the heart" multiple times, including in this battle.
From the book:
...their eyes locked. "Make it quick, Pa."
"Aye," Thornson [his father] said.
In a dreadful, tender gesture, he scooped Dennis into his arms and carried him to the river. There, he held the boy's head in the water until he stopped struggling."
Reading that again, I just have no words. This has to be a logic fail of the author's, not of Thornson.
The image of that scene is scarier than anything else in this not-zombie* story.
* The author is very big on saying this isn't a zombie story. However, in all ways but one, these creatures are zombies. Somehow they're breeding, but otherwise: zombie.