Quick answer
Both men eat what their words produce. One gets a meal; the other develops an appetite for harm.
Read both lines
The eating image runs through both halves and does the comparing. "The fruit of his mouth" is the same phrase used at 12:14, treating speech as something that yields a crop, and the man in the first line simply eats what he grew. The second line is stranger and worse. It does not say the unfaithful man is punished by violence; it says his appetite is for it, that violence is what he is feeding on. That describes a taste acquired over time rather than an outcome imposed, and it is a bleaker observation than a threat would have been.
Related: Proverbs 12:14 · Proverbs 18:20 · Proverbs 1:11–13
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.