And I am this day weak, though anointed king; and these men the sons of Zeruiah be too hard for me: the LORD shall reward the doer of evil according to his wickedness.

Wesley's Notes

3:39 Weak - In the infancy of my kingdom, not well settled in it. The metaphor is taken from a young and tender child or plant. These men - Joab and Abishai, the sons of thy sister Zeruiah. Too hard - That is, too powerful. They have so great a command over all the soldiers, and so great favour with the people, that I cannot punish them without apparent hazard to my person and kingdom; especially, now when all the tribes, except Judah, are in a state of opposition against me. But although this might give some colour to the delay of their punishment, yet it was a fault that he did not do it within some reasonable time, both because this indulgence proceeded from a distrust of God's power and faithfulness; as if God could not make good his promise to him, against Joab and all his confederates; and because it was contrary to God's law, which severally requires the punishment of willful murderers. It was therefore carnal wicked policy, yea cruel pity that spared him. If the law had had its course against Joab, it is probable the murder of Ishbosheth, Ammon, and others, had been prevented. So truly was he in these, and some other respects, a bloody man, which may be observed to the glory of the Divine grace, in his forgiveness and conversion.

King James Translators' Notes

weak: Heb. tender

Matthew Henry's Concise Commentary

3:22-39 Judgments are prepared for such scorners as Abner; but Joab, in what he did, acted wickedly. David laid Abner's murder deeply to heart, and in many ways expressed his detestation of it. The guilt of blood brings a curse upon families: if men do not avenge it, God will. It is a sad thing to die like a fool, as they do that any way shorten their own days, and those who make no provision for another world. Who would be fond of power, when a man may have the name of it, and must be accountable for it, yet is hampered in the use of it? David ought to have done his duty, and then trusted God with the issue. Carnal policy spared Joab. The Son of David may long delay, but never fails to punish impenitent sinners. He who now reigns upon the throne of David, has a kingdom of a nobler kind. Whatever He doeth, is noticed by all his willing people, and is pleasing to them.