You can tear some one down all you want, but doing that says more about you than it does of that person.
Karma is real, and when it comes back to you, it's not so pretty. Treat others how you want to be treated. Build people up. Make them feel good about themselves. Put yourself in others shoes. The girl at the cash register isn't mean-mugging you. She's having a bad day. Talk to her. Tell her she looks beautiful. Smile at her. Make her feel good.
Yes, we will all meet people in our lives that we are not the fondest of and who may try to deliberately bring you down. Ignore them. You can not fight fire with fire. Be kind to them.
Being kind is not a sign of weakness. Kindness is genuine and one of the greatest things you can give someone.
"Be so kind that if anyone were to say something bad about you they wouldn't believe it."