I tried to look at reasons people are cruel and here is what I found.
Envy. Sometimes people will try to knock you down simply because they see something about you that makes them feel insecure. In this case if there is little they can do besides fling mean words then it is actualy a compliment. No matter how great you are someone will hate you for it.
Defense or fear. There are many people that when put in a position that makes them feel bad they will blame someone or something else. This means that if they hurt you and feel bad about it they try to justify it by blaming you. Once you are to blame then it makes even more sense to that person to be cruel to you in the future. Once again the problem is them not you.
Then of course there is passed on cruelty. That is when someone has been the victim enough that they decide to victimize others. The problem here is that if they are cruel to you it rarely does any good to become cruel yourself.
So even though none of this will get rid of all cruelty maybe it can help us deal with it. When someone trys to slander you or make you feel bad, it is a person with low self acceptance who actualy thinks you are doing better then them.
When they blame you as if you deserve it, then most likely it is that person who is feeling bad. And finaly anyone who is being cruel most likely needs or needed more kidness and acceptance in their life.
If you are wondering why so many people are cruel to you, make sure your not being cruel and then just know that as long as your doing good then haters are just sad people who are lost in life.