Albert Barnes Commentary 1 Kings 2:22

"And king Solomon answered and said unto his mother, And why dost thou ask Abishag the Shunammite for Adonijah? ask for him the kingdom also; for he is mine elder brother; even for him, and for Abiathar the priest, and for Joab the son of Zeruiah." — 1 Kings 2:22 (ASV)

Ask for him the kingdom also - Bathsheba had not seen anything dangerous or suspicious in Adonijah’s request. Solomon, on the contrary, takes alarm at once. To ask for Abishag was to ask for the kingdom.

To the Oriental mind, a monarch was so sacred that whatever was brought near to him was from then on separate from common use. This sacred and separate character attached especially to the royal harem. Its inmates either remained widows for the rest of their lives or became the wives of the deceased king’s successor.

When a monarch was murdered, dethroned, or succeeded by one whose title was doubtful, this latter alternative was almost always adopted (Compare 2 Samuel 12:8; 2 Samuel 16:22). Public opinion so closely connected the title to the crown with the possession of the deceased monarch’s wives that granting Adonijah’s request would have been the strongest encouragement to his pretensions. Solomon, seeing this, assumes that Adonijah cherishes a guilty purpose, that there has been a fresh plot, that Abiathar and Joab—Adonijah’s counselors in the former conspiracy (1 Kings 1:7)—are privy to it, and that the severest measures are necessary to crush the new treason.