The fruit of his mouth - Speech rightly used is itself good, and must therefore bring good fruit.
Eat violence - i. e., Bring upon itself repayment in kind for its deeds of evil.

A man shall eat good by the fruit (a) of [his] mouth: but the soul of the transgressors [shall eat] violence.
(a) If he uses his tongue to God's glory, and the profit of his neighbour, God will bless him.

A man shall eat good by the fruit of his mouth,.... Or, "of the mouth"; either another's or his own, since the word his is not in the text; though it is supplied by the Targum, Aben Ezra, the Vulgate Latin, and Syriac versions, as by us. The Septuagint and Arabic versions render it, "shall eat of the fruits of righteousness". I should choose to translate the whole thus: "a good man shall eat of the fruit of his mouth": so Aben Ezra interprets it, "a good man shall eat"; and so the Septuagint, Syriac, and Arabic versions render it. The sense is, that a good man brings forth good things out of the good treasure of his heart by his mouth; which not only minister grace to the hearers, and are for the use of edifying to others, but also to himself; while he gives wholesome counsel and advice to others, it is of service to himself; while he comforts others, he comforts himself; and while he teaches and instructs others, he teaches and instructs himself: so a good minister of Jesus Christ, while he feeds others with knowledge and understanding, he himself is nourished up with the words of faith and good doctrine; so Jarchi refers it to a man's doctrine, and the reward of it here and hereafter;
but the soul of the transgressors shall eat violence; Jarchi interprets it,
"the delight of transgressors is violence;'' (u).
that is, what their souls desire, choose, will, and take pleasure in, even using violence, and doing mischief to others; and to the same purpose is the note of Gersom: but Aben Ezra supplies it from the former clause, as we do; and the sense is, that the same measure they mete out to others shall be measured out to them again; what they give others to eat, they shall eat themselves, even the bread of violence; see Proverbs 4:16. And this will be the case of all perfidious and treacherous ones, as the word (w) used signifies; of false teachers and cruel persecutors; and of Babylon, of whom it will he said, "reward her as she rewarded you", Revelation 18:6.
(u) "Anima cupido praevaricatorum est violentis", Gussetius, p. 524. (w) "perfidiosorum", Junius & Tremellius, Piscator; "perfidorum", Cocceius, Schultens.

By our words we must be justified or condemned, Matthew 12:37.

shall eat--that is, obtain (Proverbs 12:14).
transgressors--as in Proverbs 2:22.
violence--or, "mischief" to themselves.

2 From the fruit of the mouth of a man he himself enjoys good;
But the delight of the godless is violence.
2a = Proverbs 12:14, where ישׂבּע for יאכל. A man with a fruit-bringing mouth, himself enjoys also the blessing of his fruit-producing speech; his food (cf. βρῶμα, John 4:34) is the good action in words, which in themselves are deeds, and are followed by deeds; this good action affords enjoyment not merely to others, but also to himself. Ewald and Bertheau attract יאכל to 2b; so also does Fleischer: "the violence which the בּגדים wish to do to others turns back upon themselves; they must eat it also, i.e., bear its evil consequences." The thought would then be like Proverbs 10:6 : os improborum obteget violentia, and "to eat violence" is parallel to "to drink (Proverbs 26:6) violence (injury)." But wherefore then the naming of the soul, of which elsewhere it is said that it hungers or satiates itself, but never simply (but cf. Luke 12:19) that it eats? On the contrary, נפשׁ means also appetitus, Proverbs 23:2, and particularly wicked desire, Psalm 27:12; here, as Psalm 35:25, the object of this desire (Psychol. p. 202). Regarding בגדים, vid., above, p. 85. There are such as do injury in a cunning deceitful manner to their neighbour to their own advantage. While the former (the righteous) distributes to his neighbour from the inner impulse without having such a result in view, yet according to God's direction he derives enjoyment himself therefrom: the desire of the latter goes to חמס, ἀδικία, and thus to the enjoyment of good unrighteously and violently seized.

Shall eat - Shall receive comfort, and benefit by his wise and profitable discourses.