This child, Angel. It’s the one good thing we did together. The only good thing. You make sure to tell him that.
Darla is one of the strangest, most polarizing characters in the Buffyverse. She was introduced as a heartless, bloodthirsty vampire–a creature who fed on humans without thought or remorse. She was a monster. She was beyond redemption, beyond saving.
Except… she wasn’t. Not in the end.
Darla had the most chances at life, and she made the most of it. Her first life ended in 1609, when the Master turned her into a vampire. Her second life ended almost 400 years later–staked in an empty night club by the closest thing to the love of her life. Her third life began in Los Angeles and ended when, slowly dying from syphilis, she was turned into a vampire again. Darla’s cat-like lives ended for good when she sacrificed herself to allow her infant son (an impossibility and a miracle!) to live.
Darla was, more than any other character in Angel, simply a pawn for the Powers That Be. She was recruited without her knowledge (or will) to become a major player in the coming apocalypse–by giving birth to the son she shared with Angel.
Darla lived (and died, and lived) for over 400 years, but the only selfless act she committed–the only good thing she ever did–was love her unborn son so much she gave her (undead) life for the chance that he could live.
It seems strange to classify a monster as one of the most loving and selfless characters in the Buffyverse, but after all: when you live on the Hellmouth, nothing is as it seems.