Truth that we are all a little bit evil. We have that. Even the kindest people of our world are have sometimes urge to make BAD stuff. Crush, kill, do damage.
THEN WHY SOME PEOPLE DO MORE BAD STUFF THAN OTHERS?
Answer is stupidity. Stupid people are not aware that actions have consequances. Smart people think about future, and they know that their actions could crush their life.
HOW TO MAKE THEM THINK?
When I was a child I did BAD stuff to people and animals. I didn'd understand that my bad actions do harm to living creatures. I've understood that I do smt bad when my teacher at school said that if she see my bad behavier again she will make a notice to police.
This aknowledgement that I do something so bad that I worth to be interesting to police, make me think that I sshould be better to people and animals. Fear is great helper. Don't make cruelty be unpunished. Make them atleast fear of punisment.