Souls are one of those things where Joelle and I come down on very different ends of the spectrum. I am a Christian and I believe very much in the existence of souls. Do I believe in souls exactly the way they seem to work in Joss Whedon’s worlds? Probably not but it’s also not as far off as it could be. I do believe in the idea of souls as an eternal part of ourselves and that seems consistent with Buffy lore. When they die, everyone seems to go somewhere. There are hell dimensions, there are heaven dimensions. What the criteria is for getting into these places is a little dim but I assume there are some. There seems to be an idea of eternity, of people not simply ceasing to be.
What is the soul though? That’s where things get murky to me. Angel makes it sound like it’s his conscience; it’s the thing that makes him feel guilt for the evil he’s done. However, as Joelle said, it’s definitely not that simple. Demons like Clem or like Lorne in Angel are an indication that some of them are just fairly laid back guys. Depending on what you believe happens with Spike in later seasons (and that will certainly be many discussions), there is even some indication that demons might be capable of change with certain motivations. Is it easier for a demon like Clem who, we assume, does not need to feed off of humans? If demons are capable of change then even the slaying might be thrown into something of a moral grey area.
Someone like Anya became a demon but there’s never any indication she lost her soul in that process, to my memory. There are several instances of people becoming demons, in fact. Does the losing your soul bit only happen when you’re a vampire and you’ve had to die first? Willow’s rampage and murder happened while she was very definitely in possession of a soul. Dru says that vampires are capable of love. Granted, Dru is somewhat insane but I don’t think we’re given anything absolutely stating she’s wrong. With humans, the element of choice seems to come into play. Do demons not get choice? That’s most (though there are certainly segments that disagree) Christian theology, that demons only had one choice and they made it and that’s what made them demons, when God threw them out of heaven. However, that is clearly not the theory here.
I guess the bottom line is that the soul seems to me to just be the core of the person, for good or bad. I don’t think it stops you from evil at all and, as far as I can tell, the lack of one isn’t a guarantee that you will only do evil. It’s shaky ground and I often wish they would have explained it maybe a little bit better. The truth is that, much as I adore Joss, I just don’t think they tried to draw up a totally definitive theory and I think they played a bit fast and loose with it when it suited them.