Those who are kind reward themselves, but the cruel do themselves harm. Proverbs 11:17 NRSV
Another way to say the same thing: kindness is its own reward and cruelty is its own punishment. Perhaps it’s because kindness is fostering whatever lives and moves in this time and place and cruelty is putting every effort into maiming or killing it. Either way, it is sure to rebound on the person who is its source.