I am not 100% sure, but I believe the concept of 'the shadow' comes to us from Karl Jung. My understanding of the shadow, which is also influenced by my exposure to Buddhism, is that it is literally a place within is that is sheltered from the light of awareness. Since those things about ourselves we are most likely to tuck away in such a place are those which we have been taught (shamed into believing) are 'bad', the shadow can seem like a scary place, full of aggression and whatnot. But to the extent that we can, and choose to, bring the light of awareness to bear on the shadowy places within us, we will not be prone to surprise attacks of perhaps unwanted behaviour originating in some dark supposedly 'evil' place. Nothing original there...pretty standard psychology :)