Who told Benaiah to kill Shimei?

Solomon

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. ...

So the king commanded Benaiah the son of Jehoiada; which went out, and fell upon him, that he died. 1 Kings 2:44-46