Why people are so ready to inflict great harm on others simply because they are different or have different views or beliefs

There are many answers for this question. One of those reasons is simply because people tend to believe that they are superior to others. An example of this would be slavery. Just because they had a different skin color, white people thought they were better than colored people. It's sad that society is like this, and it will never go away unfortunately, as history does tend to repeat itself...