In general, it draws upon the insights of Carl Jung and, more specifically the key to one of C. S. Lewis' more readable books, "The Great Divorce".
The point is, when we try to be "good" simply by ignoring or supressing that which we deem to be "bad" in ourselves -- that is when it becomes "evil".
I touched upon this in what I hope was a more humorous fashion in "The House of Apes"
The reason I am so afraid is that sometimes,
when I look into my mind, I suddenly find
in the shadow of the flowers which I nourish there;
a dirty, little beast, with fangs bared...
as if it dares for me to say
that I demand that it should go away!
as if it wants to let me know
that it will follow me whereever I go.
That little monster you see, is a part of me;
a part of me and my divinity.
When I get to know it truely, as a friend,
all acrimony and division ends!
It then sublimes into a beautiful ape!
And I am no longer so afraid...
(25, May 1999)