John Gill Commentary 1 Kings 2:9

"Now therefore hold him not guiltless, for thou art a wise man; and thou wilt know what thou oughtest to do unto him, and thou shalt bring his hoar head down to Sheol with blood." — 1 Kings 2:9 (ASV)

Now therefore hold him not guiltless
Do not look upon him as an innocent person; and if he commits an offence against you, as he has against me, do not acquit him as I have done:

for you are a wise man ;
so it seems he was before the appearance of the Lord to him at Gibeon, even before his father's death he had given some proofs of it to David himself:

and knowest what you ought to do to him ;
to watch and observe him, and, if found offending, to punish him according to the rules of justice, and the laws of the land:

but his hoary head bring you down to the grave with blood ;
spare him not on account of his age, but put him to death whensoever he shall be found guilty, let him not die a natural death.