I think this has something to do with the Yin and Yang concept, which treats everything as inseparable opposites like in the case of left-right, negative-positive, night-day, cold-hot, and bad-good, etc. The two aspects should come together to form wholeness; hence, there is nothing completely yin and yang. Each aspect forms the beginning of the other. For instance, when the day ends, night follows. When people begin to see certain things as good, other things are tagged as evil.
Going back to your question, I do believe that good and evil is inherent in everyone of us, and whatever trait will manifest depends on whichever we get to nurture. If you are likely to do bad things then the likelihood of you being "evil" increases, whereas the goodness in you decreases. As in the concept of Yin and Yang when one aspect increases the other one decreases to create an overall balance of the whole.