Adam Clarke on 2 Samuel 12:8

Thy master's wives into thy bosom - Perhaps this means no more than that he had given him absolute power over every thing possessed by Saul; and as it was the custom for the new king to succeed even to the wives and concubines, the whole harem of the deceased king, so it was in this case; and the possession of the wives was a sure proof that he had got all regal rights. But could David, as the son-in-law of Saul, take the wives of his father-in-law? However, we find delicacy was seldom consulted in these cases; and Absalom lay with his own father's wives in the most public manner, to show that he had seized on the kingdom, because the wives of the preceding belonged to the succeeding king, and to none other.