John Gill Commentary 2 Samuel 3:29

"let it fall upon the head of Joab, and upon all his father`s house; and let there not fail from the house of Joab one that hath an issue, or that is a leper, or that leaneth on a staff, or that falleth by the sword, or that lacketh bread." — 2 Samuel 3:29 (ASV)

Let it rest on the head of Joab
That is, the blood of Abner, who was the shedder of it; let the guilt of it be charged to him, and let punishment for it be inflicted on him:

and on all his father's house ;
on Abishai his brother, and other relations that might be privy to the death of Abner, and advising to it, and ready to assist in it if necessary:

and let there not fail from the house of Joab ;
let there be always in his family, and of his seed, one or other of the persons described as follows:

one that has an issue ;
a gonorrhoea, which was reckoned infamous, and very impure, according to the Jewish law, and rendered persons unfit for society; see (Leviticus 15:1–33) ;

or that is a leper ;
whose disease was very loathsome and infectious, and shut him out of the company of men; see (Leviticus 13:1–59) ;

or that leaneth on a staff ;
being blind, as Aquila renders the word; or through weakness of body, not being able to walk without one; or through some disease of the feet, as the Jewish writers generally understand it; and R. Isaiah interprets it of the gout particularly: the word for "staff" is rendered "spindle", (Proverbs 31:19) ; and to this sense it is rendered here in, the Vulgate Latin, Syriac, and Arabic versions; and then the meaning is, let his posterity, or some of them, be so poor, that they shall be obliged to get their livelihood in so mean a way as by spinning; or let them be of such an effeminate disposition, as be more fit to handle the spindle, and do the, work of women, than to use the sword:

or that falleth on the sword ;
not by it honourably in the field of battle, but cowardly destroying themselves with it:

or that lacks bread ;
and is obliged to beg it:

all which David might say, not by a spirit of prophecy, but in a passion; and to show with what horror he resented the action, and how detestable it was to him, and how far it was for him to have any concern in it:

but though it was a very wicked action in Joab to murder Abner in this manner, and for the reasons he did; yet it was a just vengeance from the Lord on Abner for fighting against God, and acting against the dictates of his own conscience; for his rebellion against David, and perfidy to Ishbosheth, and for having been the cause of much bloodshed in Israel.