[Sunlight in Pylea doesn't harm vampires.]
Angel: Can everybody just notice how much fire I'm not on?
Gunn: As evil blood-sucking vampires go, how would you rate Angelus?
Wesley: Historically, as bad as they come. Especially when he was with his sire, Darla.
Cordelia: We're researching her now to see if she has some kind of resurrection powers. Maybe she's a vampire cat with nine lives, or something!
Gunn: So he and Darla together, bad combo?
Wesley: They rampaged through half the known world until Angel got his soul.
Cordelia: Imagine Bonnie and Clyde if they had 150 years to get it right.
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.