1 Kings 2:5

what Joab the son of Zeruiah did to me This one of the sons of Zeruiah had been all through his reign too strong for his uncle. See 2 Samuel 3:39: and for the rough remonstrances of Joab with David, cf. 2 Samuel 19:5-7. The slaughter of Absalom, though neither mentioned here by David, nor afterwards by Solomon, when Joab is put to death, must have been constantly present to their minds.

unto Abner the son of Ner Ner being brother of Kish (1 Chronicles 9:36), the father of Saul, we can understand how Abner became a man of much importance in the court of Saul, under whom he was commander-in-chief. After the death of Saul it was Abner who proclaimed Ishbosheth and supported him against David, until some quarrel arising Abner made overtures to David which were favourably received. During the earlier events of the war between Ishbosheth's supporters and those of David, Abner to save his own life had slain Asahel, Joab's younger brother. To avenge this death and probably also from jealousy, lest Abner coming into David's service should become more influential than himself, Joab calling Abner aside as he was departing from the court, in conjunction with Abishai his brother treacherously slew him. Owing to Abner's early course of action David might have been suspected of conniving at his murder. The exposure to such a suspicion no doubt aggravated the king's sorrow. David's lament over Abner's death is found 2 Samuel 3:33-34. He never forgave the murder, though he was unable during his lifetime to take vengeance on the perpetrator.

unto Amasa the son of Jether Amasa was a son of David's sister Abigail, and the name of her husband is elsewhere (2 Samuel 17:25) written -Ithra." Amasa took the side of Absalom when that prince rebelled against his father, but David forgave this, when Absalom had been slain by Joab, and appointed Amasa commander-in-chief in Joab's place (2 Samuel 19:13). Afterwards, under the pretence of saluting Amasa, Joab gave him a fatal wound with a sword which he held hidden in his left hand (2 Samuel 20:10).

the blood of war in peace For both these murders were committed when the opportunity had been gained under the guise of friendship.

his girdle … his shoes Both these portions of his dress must have been covered with the blood spurting and flowing from wounds inflicted at such close quarters.