We are all two things in a way, are we not? Deep in the marrow. Angel and Devil. Light and dark. The pull between the two is the active verb which energizes our lives.
In the end, we must be that thing the world demands of us. We must take the lust and the avarice and the ambition and bury them. All the alien, ugly things! All the things we really are!
Evil is the shadow that exists just outside our world, continuously searching for a way to come in. For a way to become flesh and blood. Either we wait for that day to arrive, or we can fight it on our own terms.