Rashi on Genesis
באדם דמו ישפך BY MAN SHALL HIS BLOOD BE SHED — If there are witnesses you kill him. Why? Because in the image of God [made He man] and he has destroyed the Divine image (Genesis Rabbah 34:14)
Radak on Genesis
שפך..באדם ישפך, G’d says: “I ,personally, will demand satisfaction on behalf of the slain person if there are no witnesses who can testify against the murderer, so that a human tribunal will execute the guilty person. This is the meaning of the extra word באדם; i.e. by means of the judges.