Thematic Bible: Intermarrying with, of kings considered treason

And she said, Let Abishag the Shunamite be given to Adonijah thy brother for a wife. And king Solomon answered and said to his mother, And why dost thou ask Abishag the Shunamite for Adonijah? ask for him the kingdom also; for he is my elder brother; even for him, and for Abiathar the priest, and for Joab the son of Zeruiah. Then king Solomon swore by the LORD, saying, God do so to me, and more also, if Adonijah hath not spoken this word against his own life. read more.
Now therefore, as the LORD liveth, who hath established me, and set me on the throne of David my father, and who hath made me a house, as he promised, Adonijah shall be put to death this day.