C. F. Keil and Franz Delitzsch on 1 Kings 2:25

1Ki 2:25

Solomon had this sentence immediately executed upon Adonijah by Benaiah, the chief of the body-guard, according to the oriental custom of both ancient and modern times. The king was perfectly just in doing this. For since Adonijah, even after his first attempt to seize upon the throne had been forgiven by Solomon, endeavoured to secure his end by fresh machinations, duty to God, who had exalted Solomon to the throne, demanded that the rebel should be punished with all the severity of the law, without regard to blood-relationship.