And is the show existentialist or not?  I think the answer is not.    There is a whole intrinsic logic and more than four or five different interpretations to why a demon can choose to be moral even without a soul, and even build a soul for itself, turning the soul in the Buffyverse merely a prize for redemption and not the basis of humanity.  Off the top of my head:

a.  One would be a Christian reason d’etre for a vampire wanting back his soul, or merely getting back the power of sacrificing for others, as we saw Darla do for her son.   It’s called the gift of grace, it is given by God to his chosen ones, and I explained it more in my essay here.

b.  The second would be Joss Whedon's answer in a conference where he was asked about this.  He answered that William was a more developed individual than Angel, consequently free to live an existentialist life within a predetermined universe.

c.  A third one is a logical conclusion.  Simply, Giles was wrong.  The counsel of watchers, the books they had, they all lied.  Vampires are all individuals with humanity despite their lack of soul.  They are redeemable and worthy because of their understanding and human memory and personality.   And in this case, the soul is merely a painful switch that turns on the pangs of conscience.     The lie we are fed, is given to Buffy and all slayers to stop them from becoming the “vampire murderers”.   In my opinion this would be the most challenging explanation and the logical one to the problem of the soul in non humans in the Buffyverse.

It would also account for the fact that there are humans who despite their souls choose to act against their conscience.  We are all free.  Willow, an established goody-two-shoes, comes to mind in her murdering Warren and then trying to destroy the world to boot.    We haven’t see that much meanness since Angelus' return in Season 2.

This gives the series its entire array of shadows of grey and in fact, a  whole palette of colour that made it an icon for this and hopefully for next generations of fans.

The Anima, or the final frontier of the soul in the Buffyverse

Here my fourth explanation and the one I believe in most.  

For those of you new to the theory of archetypes, C.G.Jung made the most revolutionary analysis of humankind psyche that changed psychology as much as Einstein changed Physics.

He discovered certain models that exist in the unconscious of all the members of humankind; those are psychic forces that interact and change and try to make individuals and societies a whole, a unity.   It is the ultimate searched for illumination.

Those are the archetypes.    Hundreds of books have given psychological interpretation to many aspects of human activity in politics, sociology, history, and especially, in literature. 

Since Christipher Vogler’s book, “a writer’s journey” most screenwriters have understood and even included consciously archetypes and Campbell’s hero’s journey in their works. 

This is the case too with BtVS.   The archetype figures were included both consciously and unconsciously, in my opinion.   I have heard interviews where the words hero’s journey were applied by the writers both to Spike and Buffy.

This gives me full conviction that the concept of soul in the Buffyverse is intimately linked to that of the archetype of the Anima.

The lack of soul is in fact, the absence of the Anima.    (Soul in Latin and Italian is translated literarily as anima).

Anima would be the feminine side of  a masculine psyche.   If lacking, his Self is not complete, there is not unity or health.  A whole journey should be undertaken until that missing part is found and correctly accepted, assimilated and loved.

The usual road to find the Anima is romantic love, as in a numinous experience, or in the words of a very bad poet, an effulgent experience.

In fact, the lack of anima, shown as a lack of soul in vampires would be a great reference to our modern American society, so empty of feminine principles, dominated by selfish interest, the rational conscience has lost its soul as values have become anti-values.  Evil has replaced Good in a subtle way.

The road to become a healthy individual or society again lies in change.  The actors of the change are the archetypes and they need representations in characters.   Transformation needs something to start, something that will put things upside down, a sacred event, and that can only be love, the kind of love that happens once in a lifetime.   A syzygy.   Then love is the transforming event, and Buffy is the representation of the Anima.  

We have two different cases of men who strive to find their anima.

First, we have Angel.  He has been imposed a soul, it is a curse because he is not ready for it and doesn’t embrace it.   There hasn’t been the transforming event that may unify him with it.   The rational constraint in him is very strong and when in contact with Buffy, as we see in “Innocence” he loses his soul.   And rejects Buffy at the same time.  He hates his love. He hates his Anima.  Yes he obsesses with her, with Buffy.

A deep problem in his psyche should be resolved before he can take the road to illumination.   

In the end we find Angel is incapable of finding love in any woman.   I find it impossible from this interpretation for Angel to be shanshued unless he comes to terms with Buffy as his anima.    You will see that when that time arrives, it won’t be a problem if some new enemy comes and steals his soul.  He won’t need it to be a moral individual anymore.

The second case is Spike.   As we all know, he has gone through a different road.  His problem with the anima starts with Cecily's rejection.  Something inside himself is desperate for the unity and finds none.  He takes the vampire life with a woman who has no anima to offer, another unsouled and damned vampire he tries to love, although every quality of feminine in her is corrupted and perverse.

Upon living with Buffy and seeing her, he find himself called to his own journey (as interesting discussed in the dream interpretation essay posted earlier in this Methathon).  It is for him, through the love experience that he makes his transformation and reaches for his soul, while looking desperately for his Anima in Buffy.

He finally finds it, (2) and is liberated to death after doing his self sacrifice for a noble cause, something that is for once not about Buffy, but about saving the world.   

He can do it because the Self is complete now.   

 Conclusion

BtVS use the soul has many different possible interpretations, such as existencialism, determinism, and others; all possibly valid, despite some apparently contradictory events and characters.   Starting a study of the soul metaphysics can entail many difficulties that only can be surmounted by determining the nature of the philosophy and psychological relations and facts governing its use.   

As a work of fiction, it is necesary to figure out an intuitive and overall tying-up approach which may be provided by the analysis of archetypes as the source of the metaphysic problem in the series.

Footnotes:

(1)  It is interesting to note that when a vampire is resouled, the human soul takes complete control of the demoniac soul.   So it is much stronger. 

(2)  From a canonical point of view, this could only have happened the night before the final battle.  I assume this as Joss Whedom mentioned it as the most important moment of the whole show.

Some sources:

Wikipedia

Le paradoxe de l’ame, by Diane Couisineu

All things philosophical in BtVS http://www.atpobtvs.com/index.html

The interpretation of Fairy Tales by Marie-Louise Von Franz