Sforno on Deuteronomy 21:8/ספורנו על דברים כ״א:ח׳

1כפר לעמך, Please Lord, grant atonement to Your people, on behalf of the one who has spilled the blood. This is a veiled request to bring the murderer to justice as it is not given to man to do this. Compare Ketuvot 30 “if someone is guilty of judicial execution (and human justice did not reach him) he will either be killed by the secular authorities (for something else he is accused of) or terrorists will attack him.”

2ונכפר להם הדם, it will be atoned through heavenly judgment, the heavenly agent spilling the murderer’s blood accomplishing this. It will appear in the eyes of man as if the matter had taken care of itself independently. [one cannot pinpoint why the party concerned died a violent death, not knowing what he had been guilty of. Ed.]