A good way they could have handeled this is to have started having Angelus come to the surface even without Angel loosing his soul. I mean, the differences between his two states of mind are so huge, I'd think that it's a sign of multiple personality disorder, with his loosing his human soul as the trigger. Normally, a vampire's personality isn't that hugely different from his or her personality as a human. Simply an extention of his or her inner most desires. Meaning, deep down, Angel is a sadistic mass murderer. He just needed the right trigger for his psychosis. Might have been interesting to see him really start to loose it, and have his Angelus tendancies bubble to the surface when he had his human soul. They did this occasionally, but never to the extent that they could have taken it to.