Adam Clarke on 2 Samuel 3:27

And smote him there - Joab feared that, after having rendered such essential services to David, Abner would be made captain of the host: he therefore determined to prevent it by murdering the man, under pretense of avenging the death of his brother Asahel.

The murder, however, was one of the most unprovoked and wicked: and such was the power and influence of this nefarious general, that the king dared not to bring him to justice for his crime. In the same way he murdered Amasa, a little time afterwards. See 2Sam 20:10. Joab was a cool-blooded, finished murderer. "Treason and murder ever keep together, like two yoke-devils."