Ok this is probably the biggest issue I have with this show b/c it is never made clear. What is a soul in the buffyverse?...
The concept of a soul is directly related to the idea of free will. Both Buffy and Angel are ultimately shows about power--who has it, who controls it, how the powerless can fight back. This concept is at the very core of the mythos, with vampires being a metaphor for (among other things) the idea absolute corruption--turning a human into a being of pure evil, one who has no choice but to kill and eat and seemingly take gleeful joy in it. Even when Buffy suspects Willow has been turned ("Doppelgangland") there is not a moment's hesitation: she has to kill her best friend. Willow won't have a choice in becoming an evil being. She just is one.
Angel takes that same idea and tweaks it slightly. A vast stretch of the series' run leads up to one event: the arrival of Jasmine, a supreme being who immediately makes everyone be the opposite of a vampire. The entirety of the humanity has no choice but to be good. Again: the absence of free will is the ultimate sin.
The returning of the soul is the return of free will, which is why most people act differently with it. Most, not all. Some chose to carry on doing what they were doing, choosing to be evil. Free will's tricky like that.
Well if the soul is connected to the idea of free will. Where blood thirsty demons have to kill, drink blood, or what ever and have no choice who it is and beings with souls do is nice and all but there are still examples that disprove that.
One major one comes from Angel. In the episode Lullaby Darla sacrifices herself so that her baby will live. This is an act of free will. She chooses to save the life of a living being, if she was "pure evil" she would let Connor die and "take gleeful joy in it". And you can't say it's because she is a mother because when spikes mother was turned into a vamp she had no motherly feelings towards him. And as he said it was the demon not her.
You know, I think the main point is that this universe just has no rules.
