The fruit of his mouth - Speech rightly used is itself good, and must therefore bring good fruit.

Eat violence - that is, bring…

A man shall eat good by the fruit of his mouth. —See above on Proverbs 12:14.

Shall eat violence.<…

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…

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