In my opinion our nature is both evil and good. That is to mean that we are born into this world with the capabilites of being both good and evil. Perhaps a good example to use is the yin-yang, black with a circle of white and vice versa.
Every "bad" person will have a drop of good in them, and even the holiest man will make their mistakes. Whether we make good or bad decisions later on in our life is due to nurture and that is another topic.