The king said moreover to Shimei, Thou knowest all the wickedness which thine heart is privy to, that thou didst to David my father: therefore the Lord shall return thy wickedness upon thine own head; (1 Kings 2:44)
No man knows so well all the hidden wickedness of the heart as the transgressor himself. With his life at stake, Shimei knew that his own wicked heart was the most telling witness against himself.
Thine own head.
God is not an arbitrary executioner of the sentence against transgression. Sinners reap in judgment what they themselves have sown. It was Shimei’s own iniquity, not merely the judgment of an earthly king, that condemned him to death.