I'm not certain that attitudes to killing do change in A Long Walk to Water. Guns are introduced very early in the story, as Salva hears gunfire during the first chapters when he must flee his village. If anything, killing only becomes more and more real as Salva encounters more guns.
The idea of killing becomes most abhorrent when Salva's uncle is murdered by a man with a gun. In general, the distant gunfire at the beginning of the story is terrifying, but the full visual imagery of murder via gun gives the reader (and Salva) a true sense of the horror of killing. It moves beyond a theoretical of "being forced to fight" and into the true horror of losing the person Salva depends on the most.