Albert Barnes Commentary 1 Kings 2:23

"Then king Solomon sware by Jehovah, saying, God do so to me, and more also, if Adonijah hath not spoken this word against his own life." — 1 Kings 2:23 (ASV)

Against his own life - Adonijah had forfeited his life by his former conduct, and his pardon had been merely conditional (1 Kings 1:52).