Rashi on Numbers 35:30/רש"י על במדבר ל״ה:ל׳

1כל מכה נפש וגו׳ HE WHO KILLETH A PERSON, etc. — This means: He who proposes to kill him because he had killed a person,

2לפי עדים ירצח BY THE EVIDENCE OF WITNESSES ONE SHALL SLAY [THE MURDERER] — who testify that intentionally and after due warning he killed him.