Why are mean people on earth?

The way this question is Worded makes it sound like a spiritual thing, but it's not. Mean and aggressive people posses traits that draw them to be that way, but not all people who act this way have no control over their actions and Words. Different things make different people tick (past experiences, the way they were raised etc). This question could probably be best answered by a Psychologist. Personality traits and the way we change and adapt has been a popular topic of conversation with Psychologists for years now.