A man shall eat good by the fruit of his mouth: but the soul of the transgressors shall eat violence. A man shall eat good by the fruit of (his) mouth. He shall enjoy the fruit of the good words which he uses (Proverbs 12:14). No wonder that the Holy Spirit here labours so much for the reformation of the tongue; for the apostle also (Romans 3:13, etc.), when giving an anatomy of human depravity in the members of the body, dwells more on the tongue than all the rest (Cartwright).
But the soul of the transgressors (shall eat) violence - the transgressors who seek to inflict injury on others shall suffer the fruit of injury in their own soul. The soul of the transgressors (including all their inward man) is opposed to the mouth of good man. They do not show by the mouth the evil which is within their soul; but the good man's month indicates the good heart within. Therefore his mouth is mentioned, but their soul.