Luke 6:27-28

But I say to you who hear (Me and pay attention to my words): Love (that is, unselfishly seek the best or higher good for) your enemies, (make it a practice to) do good to those who hate you, bless and show kindness to those who curse you, pray for those who mistreat you. 

We know that evil exists, and it uses people to hurt people. Those who are hurt will hurt other people, not recognizing where the problem of the pain originated from the devil. Being kind to those who mistreat others stops the cycle of harm and abuse.  When we react unkindly toward each other, we perpetuate the cycle.