Not sure that I fully understand what the question is asking, but if you want to talk about the Duality of man, then it is a theory that all men have both good and evil inside them. Perhaps it works as a balance? Perhaps like a Chinese Yin and Yang? Good source is the scene from Full Metal Jacket where Joker explains his Peace sign button on his uniform together with the written words Killer on his helmet, and states perhaps it has something to do with the Duality of Man!