12. The sin of their mouth. Construe it this way in line with the Hebrew: “Get rid of their sin, that is, get rid of them in and with their sin, so that You may at the same time destroy both, not just the sin but the people themselves too, by saving them like Your saints.” This he shows by saying, “in sin.” For He destroys the saints in His own righteousness and strength; but He destroys the wicked in their sins, by giving them over to sin, so that they might thus perish in sin. This statement is horrible beyond measure, because in the sin of their mouth they destroy themselves and all their followers by means of their destructive teachings. And God’s wrath employs none other than the sin of their mouth for the purpose of destroying them. Therefore, as He destroyed the saints “with a rod” (Ps. 2:9), that is, by means of a rod, so He destroys those people by means of the sin of their mouth, or with the sin as an instrument, so that they run into their own arrows, like Midian and the Philistines.
Luther’s works, vol. 10: First Lectures I