However bad an evil person is, that is his own problem. He is spoiling himself. Just mind your own business and it will be all right. However, if you try to mind his business, what will happen? You will become partners with him. You will also become an evil person just like him.
When you hurt people, you are really hurting yourself. When you despise others, you are actually looking down on yourself. If you treat others badly, you are in reality treating yourself badly.
An evil person who tries to hurt virtuous people can be compared to one who turns his face upwards and spits at the sky. Eventually, the spit will fall back on his own face.