“With that King Solomon swore by Jehovah: ‘So may God do to me and add to it if it was not at the cost of his own life that Adonijah made this request […]'”(1 Ki. 2:23)
Adonijah, who was apparently the eldest of King David’s remaining sons, had already tried to make himself king once (1 Ki. 1:5-10).
David, however, had promised the throne to his son, Solomon (1 Ki. 1:29,30).
Once crowned, Solomon gave this warning regarding Adonijah: “If he behaves in a worthy manner, not a single hair of his will fall to the ground; but if what is bad is found in him, he will have to die,” (1 Ki. 1:52).
Some time after David’s death, Adonijah approached Bathsheba, Solomon’s mother, and complained, “You well know that the kingship was to become mine, and all Israel expected me to become king; but the kingship eluded me and became my brother’s, for it was from Jehovah that it became his. But now there is just one request that I am making of you. Do not turn me away,” (1 Ki. 2:15,16).
Adonijah then proceeded to manipulate Bathsheba into asking Solomon to allow him to marry David’s nurse (1 Ki. 2:17).
At this point in the story, I thought- Did Adonijah actually deserve to die just because he wanted to marry his late father’s nurse?
Upon further investigation, I learned that David’s nurse was actually viewed more as his concubine, even though he’d had no sexual relations with her (1 Ki. 1:1-4).
There was an ancient Middle Eastern custom that the wives and concubines of a king should only belong to his heir after his death (Insight on the Scriptures, vol. I: “Abishag,” par. 2; “Adonijah,” par. 4).
That is why King Solomon viewed Adonijah’s request as yet another attempt to usurp the throne which God had rightfully given him (1 Ki. 2:24,25).
In other words, it was a deceptive scheme masking high treason, and since it undermined King Solomon’s first warning and authority, it merited death.