Darla on Buffy is so much different to Darla of Angel.
On Buffy, she wasn't very scarifying, so it doesn't really matter to me, that Angel killed her. Her departure from Buffy wasn't a big deficit for the series.
But on Angel, she has been introduced completely new and different to the fans. The story about her resurrection was quite interesting and also the view back in her past (and also in her time with Angelus). She developed to an interesting character, who dominated big part of season 2. Her appearance in season 3 ended up quite surprising for me, cuz I expected everything from a pregnant vampire, but not that her child does have a sou, what makes her emotional like a human...her end was quite touching, but for sure, it would have been interesting too, if we could have seen, how she would have behaved after the birth of Connor....?! I mean, okay, it's her child, but she has been a vampire without a soul......