Personality fragments itselfe to deal with adverse situations. This applies to all the conflicts you can imagine. One character is more libidinous, more seductive, while another is extremely shy, unable to look at other people. And the personalities take turns according to the situation. If you need to be aggressive, if you need to be peaceful, and so on.. Life is it!
There is no black or white in our mind. Our experiences make us add a lot of information inside a box. We are what we remember ourselves. Can I remember who I am?!
The series can be observed as a poetic and imaginary form; rather than directly describing realism by exploring the relationships between the protagonist's personalities.
"The duplicity of my character was profound. Many men would have proudly confessed certain mistakes. But I could only be ashamed of these irregularities in view of the high purposes which I was aiming at: I concealed them with a morbid sense of guilt and shame. Thus I demanded the nature of my aspirations, rather than the degradation of my sins; more than in most mortals, this deep gap separates good from evil and divides and composes the duality of our soul. [...] Each day the two parts of my intelligence, moral and intellectual, drew me more and more to this truth, whose partial discovery was condemned to such a dreadful wreck: that man is not really one , but double." (Stevenson: The Physician and the Monster, 1886).