Why is there darkness in humanity?

I feel that there is darkness in humanity because of human nature. Because of human's 'advanced thinking' we are not as simplistic as other animals. With our ways of thinking we feel the 7 deadly sins, pride, lust, anger, sloth, gluttony, acedia and mortal sin. Those all contribute to the darkness that there is in humanity. Because we are human we are weak to our own minds. To crave what we cannot have, to seek to be higher than others, to fulfill our wants. There is darkness in humanity simply because there are humans.