A man who is kind benefits himself,
but a cruel man hurts himself.
The contemporary way of understanding the principle that is behind this proverb might be found in the phrase "what goes around, comes around." Life is remarkably reciprocal. Given enough time, good deeds become their own reward and deliberate hurts against others return to hurt the perpetrator.
This proverb centers its attention on how we treat others. It promotes kindness to other people as a great personal character trait. Kindness is a social virtue that builds good relationships and benefits everyone, including the person who is kind. Kindness is thus its own reward. That does not mean that every act of kindness will immediately be met with thankfulness and altruism, but it does mean kindness is emotionally and personally satisfying. It is wise then to be kind to other people.
The opposite of kindness in this short saying is cruelty. The word means to be abusive or even terrible with people. This is a person whose actions use and hurt people deliberately. And ultimately these actions hurt the cruel person. He may immediately get what he wants by force or through mean control, but there will be no social grace flowing back to him. And so a cruel person must keep resorting to these tactics. A wake of mistrust and distant relationships builds around the cruel person as others attempt to protect themselves. The cruelest people often end up alone, in bitter anger, sadly dysfunctional.
I want to be a kind man with a generous soul, ready to invest well in others because God loves them. God forgives repentant sinners in the ultimate act of kindness, offering lavish grace to them that He alone provides since they can do nothing for themselves. That's my model of kindness. God did this for me, a ruthless sinner who was headed toward cruelty. And if God did this for me, then through His love, I should be kind to others.