When Kira returns from attending her mother's corpse on the Field of Leaving, she intends to rebuild her burned cott. The women of the village, led by Vandara, want to use that land to build a pen for their children and chickens. They confront Kira, some with rocks in their hands, and tell her that she doesn't deserve to live because of her disabled leg. Kira knows that she cannot appeal to these hard women on the basis of compassion or morality. She can only reach them by putting fear into their own hearts—fear that will outweigh their brutality.
She reminds them that conflicts that could result in death must be brought before the Council of Guardians. If a death occurs in such a conflict when the guardians haven't been consulted, then "the causer-of-death must die." What Kira and the law she quotes are referring to is what we would call murder. Murder and murderer would be more succinct ways to describe the issue, but the community evidently does not use these words.
Why would the culture refrain from using those terms? Kira's society places a very low value on human life, and euthanasia and infanticide are widespread and accepted as normal. Infants with disabilities are killed, and it's considered acceptable for any infant to be killed in the few days after birth before its spirit is thought to come into its body. A person who becomes injured to the point where it interferes with his or her ability to work may also be killed. Kira learns of a woman with five children who broke her arm and will probably be killed. Since their society practices murder on a regular basis, perhaps the word was removed from their vocabulary because it seemed too severe.