“avenger of blood.” The ancient world did not have a police force to keep people safe. The best personal safety came from being a member of a large and powerful family, clan, or tribe that would seek revenge if anything happened to one of their members. If a family member was harmed, the person who avenged that family members was called “the avenger of blood” (cp. Num. 35:19-29; Deut. 19:6-12; Josh. 20:2-9).
The problem with the system was that, as has often happened in trials throughout the millennia, different people feel very differently when someone is killed. To some the killing was justified, to others it was unjustified murder. So one person would kill another. Then the avenger of blood from the dead man’s family would kill the killer. Then an avenger of blood from the family of the man just killed would kill that person. And so the pendulum would swing back and forth with people killing one another, and “blood feuds” would often last many generations.
In a case that made it before the king, like in the case of the woman in 2 Samuel 14:1-11, the king could command the killing to stop, but that was only marginally effective because the king’s command did not stop the animosity, which could erupt at any time.