Quick answer
Two failures with hatred: concealing it behind a smiling face makes you a liar, and venting it as slander makes you a fool. The proverb closes both exits.
Read both lines
Most proverbs in this chapter set a wise line against a foolish one; here both lines are failures. A man with hatred in his heart can hide it, smile at the person he despises, and the proverb calls that lying lips, a life of daily small theater. Or he can discharge it as slander, and that is folly, because slander always ricochets. The unspoken third option is the point: if concealing hatred is lying and spreading it is folly, the only wise move left is ending it, the honest conversation, the covering love of 10:12, the release of the grievance. Hatred has no good storage form.
From this side of the cross
Forgiven people have somewhere to take hatred besides the basement or the megaphone. You can afford to put a grievance down; nothing about your standing was ever funded by winning it.
Related: Proverbs 10:12 · Proverbs 26:24–26
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.