Angel was a character that made it through an amazing 8 seasons (i.e. longer than any single show) of the Whedonverse (Buffy the Vampire Slayer and Angel), and the dualism of his personality was always something that both fascinated and puzzled me. It seemed to change over time, starting off with it being basically that Angel just himself turned evil, and later Angelus morphed into a sort of separate character who was just trapped passively in Angel's body most of the time.
I must say I prefer the original concept, of Angel basically being a bad drunk who turned into a wholly unpleasant person without his sobriety/soul, and was wondering what people thought was the reasons for the change. Was it just that the Angel/Angelus concept didn't really change, but that the other characters chose to see them as separate entities in order to forgive Angel for what he did, or was it something that the show creators did unwittingly, perhaps so that we as viewers would forgive him more easily?