John Gill Commentary 2 Samuel 13:28

"And Absalom commanded his servants, saying, Mark ye now, when Amnon`s heart is merry with wine; and when I say unto you, Smite Amnon, then kill him; fear not; have not I commanded you? be courageous, and be valiant." — 2 Samuel 13:28 (ASV)

Now Absalom had commanded his servants
Before he and his guests were set down to the entertainment:

saying, mark you now when Amnon's heart is merry with wine ;
as he was determined to make him if possible, and as he supposed he would be, knowing his inclination to drink:

and when I say unto you, smite Amnon, then kill him ;
smite him that he die, and be sure he is dead before you leave him:

fear not, have not I commanded you ?
who am your lord, the king's son, and will then be heir to the crown; fear not, I will protect you; let all the blame be laid to me, if any; I will be answerable for it, you have nothing to do but to obey my commands:

be courageous, and be valiant ;
show yourselves to be men of courage and valour, not fearing the king's sons, or any in company, or what will be the consequences of it; do your business effectually, and leave all with me; it is very reasonably supposed that Absalom had not only in view to revenge the rape of his sister, but to get himself next heir to the crown.