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.

The Hebrew word neh-phesh, rendered “soul” in many Old Testament passages, is here rendered “life,” for Adonijah’s own life is at stake.
Adonijah had been pardoned at first, but a later offense and insult to the king led to his execution a little after Solomon was crowned king, for his word grew out of unholy ambition to take advantage of David’s old age to take the throne.
The immediate occasion was his request for Abishag, which Solomon treated as a plot against his life and kingship, since by seeking the former king’s concubine Adonijah could strengthen his claim to the throne among the people.
In response to that attempt to secure the throne, Solomon swore an oath by the LORD and passed sentence that Adonijah had spoken this word against his own life and would die for it.

The same judgment involved others around the throne: Joab failed in loyalty to his king and was put to death for disloyalty, Nathan the prophet knew the Lord’s will and the king’s mind and interfered with Adonijah’s plan, and Shimei was put to death for failing to abide by his conditions of life after cursing David.
Adonijah’s attempt to take David’s throne ended in his own overthrow and death as part of the continuing judgments that came upon David’s house and later history after his sin involving Uriah.
That overthrow fulfilled Nathan’s denunciation and shows that sin brings consequences of its own kind.