Quick answer
Kindness and cruelty both land back on the person practicing them. The verse follows the return path rather than the outgoing one.
Read both lines
Both lines point at the actor rather than the recipient, which is the surprise. Mercy is usually described by its effect on the one receiving it; here the effect named is on the one giving it, and the word is nephesh, a person's own life and self. The second line makes the reverse just as physical, with cruelty troubling a man's own flesh. The claim is not that kindness is a strategy for self-benefit, which would spoil it. It is an observation that how a person treats others shapes the person, and that no one administers cruelty without absorbing some.
Related: Proverbs 11:25 · Proverbs 19:17 · Matthew 5:7
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.