2 Samuel 3:39

And I [am] this day weak, though anointed king
Which seems to be given as a reason, or for an excuse why he did not inflict just punishment upon the murderer, according to the law of God, because he was "weak"; not in body or mind, but with respect to the kingdom, that was like a tender branch, or in its infant state; and great care and caution were to be used that it was not overturned: he was a king by unction, not by birth; a son of the late king was yet up against him, and was possessed of the far greater part of the kingdom; he was indeed anointed by Samuel to be king over all Israel; but as yet he was not put into the possession of the kingdom he was anointed to; he was anointed and made king over Judah, and invested with the office of king there, and settled in it; and yet his power was not very great there, for as follows:

and these men, the sons of Zeruiah, [be] too hard for me;
his sister's sons, Joab and Abishai, they were a check upon him; he could not do what he would, their influence was so great, both in the court and in the camp; the one was general of the army, and the other a considerable officer in it, and both variant men, and very respectable among the people, for their achievements in war, and the success they had; so that they were very much out of the reach of David to bring them to justice, without shaking his kingdom; and therefore in point of prudence he thought it best to connive at this fact until he was more established in the kingdom. Whatever may be said for this conduct, it is certain he was too dilatory, and which did not sit easy upon his mind, and therefore gave it in charge to Solomon before his death not to suffer Joab to go to his grave in peace, ( 1 Kings 2:5 1 Kings 2:6 1 Kings 2:31-34 ) . Some take these words, "weak" and "hard", in a different sense, that David was weak or "tender" ^F24, as it may be rendered, tenderhearted, of a merciful disposition, and therefore spared Abner when he was in his hands, though he had done him so much harm, who was the Lord's anointed; but these men, his sister's sons, were of cruel tempers, more unmerciful than he, and therefore slew him; but the first sense seems best:

the Lord shall reward the doer of evil according to his wickedness;
which may be considered either as an imprecation of evil on Joab, or a prediction, that sooner or later righteous judgment would be rendered to him by the Lord; with whom he leaves it to take vengeance on him, satisfying himself with this for the present, that though it was not in his power to do it, the Lord would in his own time and way: but after all that can be said in favour of David, he seems to have been too much in fear of men, and too distrustful of the power and promise of God to establish him in his kingdom, and was too negligent of public justice; which had it been exercised, might have prevented other sins, as the murder of Ishbosheth, to which the authors of it might be encouraged by this lenity.