Matthew Henry on 2 Samuel 3:7-21

Many, like Abner, are not above committing base crimes, who are too proud to bear reproof, or even the suspicion of being guilty. While men go on in sin, and apparently without concern, they are often conscious that they are fighting against God. Many mean to serve their own purposes; and will betray those who trust them, when they can get any advantage. Yet the Lord serves his own designs, even by those who are thus actuated by revenge, ambition, or lust; but as they intend not to honour him, in the end they will be thrown aside with contempt. There was real generosity both to Michal and to the memory of Saul, in David's receiving the former, remembering probably how once he owed his life to her affection, and knowing that she was separated from him partly by her father's authority. Let no man set his heart on that which he is not entitled to. If any disagreement has separated husband and wife, as they expect the blessing of God, let them be reconciled, and live together in love.