What my t told me is that when we experience a part of ourselves as evil, it is an introject...we have taken into ourselves some of the terrible messages we've been given about ourselves by people who mistreated us.

Do the things that "evil" one says to you sound like something that a person has said about you in the past? If you were told bad things about yourself, you may have believed it, and now you experience those bad messages as coming from you...when they may come from how other people treated you.