Why people are so mean???

I know I have to live in this world no matter how mean people are. In other words, living in one community, you have to bare with all the attitudes, even when deep inside,you fell like killing that person, slap her in the face, smack her head to the wall. Well, some people may not know that they hurt you so much, what matters to them is what they feel.they dont care if you sad or not, if you are happy with his decision or not. Even when she knows that she hurt your feeling, she will not give a damn.that is how mean people are. This kind of people is a spoil brat. They will always get what they want. This people always think that they never hurts others feelings but in fact they are so dumb and spoiled that they never really care about others.I think, she will not survive in any difficult situation. Stupid. Just now I went to surau, Ustaz said that, "what you did to others,others will do it to you".It just like karma,God have His own way to teach His people. I believe in that, even being passive and ignorance, i know that God always be with me. He will help me.He will answer my prayer, InsyaAllah. The person is what she's are. She is herself. We cannot change that. Even she is the meanest,dumbest,pain in the ass and foolish person you ever met.And I know someday, the person will feel the pain that I feel right now.