Adam Clarke on Numbers 25:4

Take all the heads of the people, etc. - Meaning the chiefs of those who had transgressed; as if he had said, "Assemble the chiefs and judges, institute an inquiry concerning the transgressors, and hang them who shall be found guilty before the Lord, as a matter required by his justice." Against the sun - in the most public manner, and in daylight. Dr. Kennicott has remarked that the Samaritan and Hebrew texts must be both taken together, to make the sense here complete: And the Lord said unto Moses, Speak unto all the heads of the people; And Let Them Slay The Men That Were Joined To Baal-Peor; and hang them up before the Lord against the sun, etc.