When Joab learned. Lit. they told him) that Abner had been with David, and he had sent him away again, he went to David to reproach him for having done so. "What hast thou done? Behold, Abner came to thee; why then hast thou sent him away, and he is gone quite away?" i.e., so that he could go away again without being detained (for this meaning of the inf. abs., see Ewald, 280, b.). "Thou knowest (or more correctly as a question, Dost thou know?) Abner, the son of Ner, that he came to persuade thee (i.e., to make thee certain of his intentions), and to learn thy going out and in (i.e., all thine undertakings), and to learn all that thou wilt do" (i.e., all thy plans). Joab hoped in this way to prejudice David against Abner, to make him suspected as a traitor, that he might then be able to gratify his own private revenge with perfect impunity.

2 Samuel 3:30 Parallel Commentaries

slew Abner

Proverbs 28:17 A man that does violence to the blood of any person shall flee to the pit; let no man stay him.

Acts 28:4 And when the barbarians saw the venomous beast hang on his hand, they said among themselves, No doubt this man is a murderer, whom...

because

2 Samuel 2:19-23 And Asahel pursued after Abner; and in going he turned not to the right hand nor to the left from following Abner...