Thematic Bible: Intermarrying with, of kings considered treason

So she said, "Let Abishag the Shunammite be given to Adonijah your brother as a wife." King Solomon answered and said to his mother, "And why are you asking Abishag the Shunammite for Adonijah? Ask for him also the kingdom--for he is my older brother--even for him, for Abiathar the priest, and for Joab the son of Zeruiah!" Then King Solomon swore by the LORD, saying, "May God do so to me and more also, if Adonijah has not spoken this word against his own life. read more.
"Now therefore, as the LORD lives, who has established me and set me on the throne of David my father and who has made me a house as He promised, surely Adonijah shall be put to death today."