John Gill Commentary 1 Kings 2:8

"And, behold, there is with thee Shimei the son of Gera, the Benjamite, of Bahurim, who cursed me with a grievous curse in the day when I went to Mahanaim; but he came down to meet me at the Jordan, and I sware to him by Jehovah, saying, I will not put thee to death with the sword." — 1 Kings 2:8 (ASV)

And behold [you have] with you Shimei the son of Gera, a
Benjamite of Bahurim
Of whom, and of his native place, (See Gill on 2 Samuel 16:5); he was now at Jerusalem, and so with Solomon, or near at hand; and was on his side, and of his party; see (1 Kings 1:8) ; but not to be trusted, or looked upon as a real friend:

which cursed me with a grievous curse, when I went to Mahanaim ;
a place on the other side Jordan, of which place, and the curses this man cursed David, with, see (2 Samuel 16:7 2 Samuel 16:8) (17:24) ;

but he came down to me at Jordan ;
after the defeat of Absalom, and when David was returning, and humbled himself to him, and begged his pardon:

and I swore to him by the Lord, saying, I will not put you to death
with the sword ;
but this oath was not binding upon his successor, and especially should he commit a new crime.