Why is there darkness in humanity?

The definition of humanity is the quality of being human. Human nature is usually described as benevolence, kind, caring, et cetera, but wars and violence are also a very big part of humanity. That brings up the question: Why is there darkness in humanity? The literal meaning of darkness is the absence of light, but some further meanings are wickedness or evil, obscurity or concealment. If we turn on the news, chances are they will say something about shootings. This is totally darkness, as anything evil is considered dark also. All the time, we have wars, violence, crimes, thievery - these are all very dark, and they are very well part of human nature. But the real question: Why is it a part of human nature? One may argue that the society is unfair to the citizens, and therefore the citizens rebel by committing crimes. Also, darkness could be used to bring out the justice system. Without bad, there cannot be good. Regardless of the reasons, darkness exists, and it is a big part of human nature.