1 Kings 2:37, commentary and pictures

The command belongs to Solomon carrying out, after David's death, his father's charge about securing the throne. It stands in Solomon's settling of the kingdom at the beginning of his reign, when Adonijah fled to the horns of the altar and was pardoned for the time being and later put to death for a further offence and insult to the king, Joab was also put to death, and Abiathar the priest was retired from his office. To Shimei, who had cursed David, it gave a clear warning and condition of life: if he left Jerusalem and went out and crossed the brook Kidron, he would know for certain that he would surely die, and his blood would be upon his own head by his own fault for breaking the king's command.

The restriction put Shimei's life into his own hands and tested whether he would honor his solemn oath, and his later journey to Gath showed his disregard for that oath. The phrase on the day does not mean he would die that same day, but that crossing the Kidron in defiance of the command made his death certain and inevitable from that day onward. The day the sentence was given fixed the penalty though its execution came later, which illustrates the difference between a death sentence taking effect and the death itself being executed, a meaning confirmed by Solomon's later restatement of the law. In the same way the warning to Adam meant that the day of transgression brought a sentence of death rather than immediate death.

Shimei failed to abide by that condition and was put to death after Solomon was crowned king, when Adonijah and Joab were also slain, receiving the just consequence of his wickedness. The account shows that Shimei, who insulted David while supposing David was falling, was the one who ultimately fell, and David was not brought down by his attacks. The verse belongs to the closing history of David that shows his great wickedness in the case of Uriah was not covered up or excused, but was followed by judgments in which Nathan's terrible denunciation was completely accomplished.