If he turn not, he will whet his sword— Since he will not turn back, he is whetting his sword, i.e. "Since this particular enemy of David's will not turn, &c." The Psalmist is supposed, to see God preparing instruments of vengeance against his pursuers, since they would not turn from pursuing him, nor cease to lay snares for his life.
Related Commentaries of Psalms 7:12
Psalms 7:12
12 If he turn not, he will whet his sword; he hath bent his bow, and made it ready.