1 Kings 2:23

Then king Solomon sware by the LORD, saying, God do so to me, and more also, if Adonijah have not spoken this word against his own life.

God do so to me, and more also. The common form of introducing a solemn oath. Adonijah ... spoken this word against his own life. Whether there was a treasonable design concealed under this request or not, the act, according to Eastern notions, was criminal, and of dangerous consequence to the state. There is no ground of censure upon Solomon for cruelty or precipitation in this instance. He had pardoned Adonijah's former conspiracy; but this new attempt was rebellion against the viceroy appointed by the Divine King, and called for condign punishment. The office of executioner was, among the Hebrews, as in other ancient countries of the East, performed unceremoniously and privately-often without any previous warning-by the captain of the guard, or one of his officers (Matthew 14:10).