Guilty Heart

1 Kings 2:44
The king also said to Shimei, "You know in your heart all the wrong you did to my father David. Now the LORD will repay you for your wrongdoing.

This comes at the end of the chapter that began with David's last words to Solomon. The narrative moves right through Solomon's carrying out his requests, executing first Adonijah, then Joab, and finally Shimei, though he got a reprieve of 3 years.
In theory, he could have stayed alive if he had kept the terms of his house arrest, but, maybe going a little stir-crazy, he jumped on his donkey one day and headed for Gath after some runaway slaves. Now surely he had another slave he could have sent after the escapees. But he took a donkey ride, and probation was over.
His crime was cursing the king as David walked his trail of tears out of Jerusalem. When the king returned, he declined to take his revenge. But evidently he never quit thinking about it, and he made it his last request.
Did Solomon find that duty more difficult than eliminating his over-ambitious older brother and the likewise over-reaching General Joab? Shimei was a fool at worst, no threat to the throne. Maybe that's why Solomon gave him a chance. When he blew it, the king could avenge his father's dignity with a clear conscience.
First he reminded Shimei of his crime. He suggested that the wrong he'd done lived deep in his memories, an ever-present reminder of guilt.
And then he gave the order to Benaiah.