I agree that the soul in the Buffyverse stands in for a conscience. Here's where I found this to be true: In Dopplegangland, when Buffy was trying to tell Willow that Vamp Willow wasn't really HER anymore but Angel was like "Well, actually..." And Buffy had to shut him up. The whole "It's just a demon, it's not really your friend/loved one" is just the comforting lie that Watchers tell their slayers (I assume) to make them do what they must, even when someone they love stares them in the face. And Buffy, I really believe, is the slayer who sees past that, but knows how important that lie is to tell her friends. This explains Spike's behavior sans soul. He's still very much William, the man who was a romantic at heart. The soullessness in him, the demon part of him just helped him find a romantic side to his new way of life and kept him from feeling remorse. I would even say that WITH a soul, he would have tried to vamp his mother, because he did love her dearly and didn't want to see her suffer in illness any longer.
This also explains Angelus. He never really loved anything (except his sister, Kathy) and spent his life trying to rebel against his parents and the way of life he was EXPECTED to have in Galway. He wanted more than anything to prove to his parents that he was strong and independent, and didn't need his father to control his life. I think he cared for his father enough that all the ways his father tore into him really did hurt. After being turned, he wanted to prove once and for all that he was better than his father, more successful than him. Being a vampire gives you this inflated ego. You're immortal, you're strong, you can have whatever you want, and in a vampire's eyes it IS success. Hence him killing his family first. I think his mother and sister were just collateral damage to make a point to his father. He may have still loved them, but the lack of a conscience/soul meant that their deaths would just make the impact on his father more hurtful and effective.