Revealing mistake: In the last scene of the episode, Angel kicks the guy out of the glass building. This guy's a vampire, and you can see his reflection in the glass windows. Vampires aren't supposed to have reflections.
Continuity mistake: When Angel kicks the vampire out the window, he does it with little effort and movement, but when the shot goes from outside the building, he's swinging his body like he's put more into it.
Answer: Due to his epiphany, Angel basically had the 'true darkness' he worked so hard to reattain ripped away from him, like his soul was after sleeping with Buffy.