שלוש ערים תבדיל, after Moses had explained the function of the judges, the king, the priests, and the prophets, Moses now details some of the tasks that fall under the jurisdiction of the judges, among them the establishment of the cities of refuge, the boundaries of the land, the examination of witnesses to establish that their testimony is truthful and accurate, and that they are acceptable as witnesses. Next, he deals with the tasks that fall within the competence of the king, such as leading the nation in war, especially aggressive wars beyond the boundaries of the land of Israel. (20, 1-9) The priests also have a role to play during the preparation of what are termed מלחמת רשות, “halachically permissible wars,” although they are not fought in order to repel invaders. The priests and judges cooperate in some of their duties, especially the עגלה הערופה, the calf killed when a murder in the field was unresolved. (21,1-9)

ובערת דם נקי מישראל, the penalty for spilling innocent blood in the Holy Land is normally the shedding of the murderer’s blood, which alone brings atonement, no substitutes being allowed, such as financial reparations (compare Numbers 35,33)

וטוב לך, because the murderer cannot kill additional people, the ones surviving the execution will be awed by having heard or witnessed the execution. (verse 20)

והנה עד שקר העד, שקר ענה באחיו. His testimony was deliberately false, meant to incriminate his fellow Jew. He did not err in what he thought he had witnessed, such as there having been poor lighting on a cloudy day. Neither did he err in the day of the month when the supposed crime occurred, having been unaware that the most recent month had been extended by a day.