I have a slightly different take on the whole 'soul' issue. I think that once humans are turned into vampires their emotions and desires are distorted by the violent nature of the demonic force now present in their bodies. Take Liam, for example. We don’t know much about him, but from the glimpses we got it seemed like he was very frustrated by living in a small village - he wanted to explore the world which, initially, was what drew him to Darla. Angel is the same. He is interested in art, for example, which implies both a curiosity about culture and a fascination with things of beauty. Cut to Angelus who is obsessed with the idea of murder and torture as an art form (“A good kill. It take pure artistry”). My theory is that, once the soul is removed, whatever Angel feels passes freely through a “demonic filter”. Liam resented his father; Angelus murdered him. Angel uses his understanding of people to help the helpless; Angelus uses his knowledge of people to break them, mentally. Angel loves Buffy; Angelus was obsessed with her. Angel’s love for Buffy was warped into something twisted and sadistic once the soul left his body. Angel still exists within Angelus, but his opinions, thoughts and desires are voiced viciously through something he has no control over. That’s part of the gypsies punishment - if Angel experiences true happiness, he will be forced to watch himself destroy everything he’s come to love.