Too many separated Angel and Angelus, mistaking them as different beings. They assumed that when Angelus was in control, Angel was gone, and vise versa. They imagined Angel and Angelus as two connected, but still dissimilar individuals, the soulless monster, and the cursed man, doomed to walk in a beast's skin.
Angel guessed that the main reason for the confusion was the nature of the soul. What was it, exactly? Did any mortal know for sure? They undoubtedly assumed that when the vampire had a soul, he or she was a different person entirely. After all, a soul was the quintessential essence of a person, so, by giving a monster a soul, wouldn't, by the same school of thought, a new person be inserted into that body? Perhaps. But although Angel would often go off in circles wondering who he was, he knew who he wasn't. He wasn't Liam. Liam, the boy whose memories had fueled the beast that had stolen his skin, was gone. His soul, the same soul that should have returned to his body at the moment the curse had taken hold, had not graced his old body with his long-absent touch. Liam had died centuries ago, in an alley, friendless and devoid of purpose and plan; sired by the vampire he had followed with so guilelessly, and so willingly, with nothing but alcohol and lust in his heart. Liam had died, and had left his body, never again to return.
Angel was not Liam. So, then, who was he? The soul that had been forced upon him, whose was it? Simple, he did not have a soul. Since the moment of since he had woken, terrified and confused in his own coffin, he had been soulless. It had not been a soul that had been granted to him by the gypsies, it had been regret. Not a soul they had given him, but regret, deep and raw, making everything else secondary, his playful malice, his sadistic glee, pushing them into the recesses of him, so far back that they almost vanished.
Almost. And only at the moment of pure joy, uncontaminated with the regret that was oh-so-hard to detach from, that those areas of himself were allowed to burst to the surface, were given free reign, and allowed to indulge themselves once again.
Angel and Angelus were not separate beings, Angel reasoned, they were the same. In a sense, Angel thought, he was Angelus at every moment- an Angelus filled with despair and regret. The soul had never been part of the equation, only artificial mourning and false sorrow. Angel wasn't even real. There was only Angelus.