Adam Clarke on 2 Samuel 3:13

Except thou first bring Michal - David had already six wives at Hebron; and none of them could have such pretensions to legitimacy as Michal, who had been taken away from him and married to Phaltiel. However distressing it was to take her from a husband who loved her most tenderly, (see 2Sam 3:16), yet prudence and policy required that he should strengthen his own interest in the kingdom as much as possible; and that he should not leave a princess in the possession of a man who might, in her right, have made pretensions to the throne. Besides, she was his own lawful wife, and he had a right to demand her when he pleased.