Because the cruel impulse has been a break of bond with others where pledge once lived.
The only overt act that can bring pain to self is that cruel act which then transgresses things to which we had agreed.
Share action with a group or person in your life, agree to mutually survive by some specific code and then be cruel to them and so transgress and you’ll have pain.
All mankind lives and each man strives by codes of conduct mutually agreed. Perhaps these codes are good, perhaps they’re bad, it’s only evident they’re codes. Mores bind the race.
Coaction then occurs. Thought and motion in accord. A oneness then of purpose and survival so results.
But now against that code there is transgression. And so because the code was held, whatever code it was, and man sought comfort in man’s company, he held back his deed and so entered then the bourne in which no being laughs or has a freedom in his heart.