Sforno on Deuteronomy 19:13/ספורנו על דברים י״ט:י״ג

1ובערת 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)

2וטוב 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)