2 Samuel 3:39 - Matthew Poole's English Annotations on the Holy Bible

Weak, or tender, in the infancy of my kingdom, not well rooted and settled in it. The metaphor is taken from a young and tender child or plant. The sons of Zeruiah; Joab and Abishai, the sons of my sister Zeruiah. To hard for me, i.e. two 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 yet in a state of opposition against me. But this, although it might give some colour to the delay of their punishment for a season, yet it may seem to have been one of David's infirmities, 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, or would not, make good his promise of the kingdom to him, without and against Joab and all his confederates; and because it was contrary to God's law, which severely requires the punishment of wilful murderers, Genesis 9:6 Exodus 21:14 Numbers 35:21, which David had no power to dispense with. And David might and should have remembered how dear Saul paid for this very thing, that he dispensed with God's command, and spared these whom God commanded him to slay, 1Sa 15. And it seems David's conscience oft smote him for this; which made him watch for a fit opportunity to remove, and then punish him, and having neglected it till death, he declareth his sorrow for that neglect, by giving Solomon a charge to execute it after his death, 1 Kings 2:5,6,34.