To me, Darla's suicide was more than just a selfless act. It was the first time Darla has ever cast off her chains and taken her own direction.
Darla was a prostitute originally, living for someone else's profit. Then the master claimed her, and she lived in servitude to him. Then she jumps at the chance to follow angel, but in reality is just trading one master for another, albeit younger and more inspired master. Angel was the driving force of their destiny, and Darla was along for the ride. Angel get's his soul back, and she is literally in tears. She has lost her direction again, and no longer can fool herself into feeling like she has some power over her own destiny.
She returns to the master, and serves him again, though with perhaps a sad air of wistfulness to her, a sense of resignment and loss, as if her spirit was broken. She dies. She is ressurected and once again is the puppet of someone else's plans. Being Darla, and having lived in servitude all her unnaturally long life, she strikes out in anger against W&H and murders the people she sees as trying to control her. At this point in time, I can really see the will in Darla to finally be free from her chains. She is sick of being someone else's puppet, and wants to be master of her own destiny. However, her freedom is, once again, only an illusion. The W&H execs she killed were dancing on strings of their own, and Darla is still not free. She gets pregnant, and is forced to wander with the knowledge that she is still, as she has been all her life, a pawn.
Her final act is to, for the first time, take 100% control of her life in the ending of it. She is finally free from control, and in the process, her action saves the baby she has come to love. It took the love of a mother to give her the strength to finally take charge of her own life, and it was the end of her. But therein lies the true tragedy, as even that act was not without influence, as we discover in Season 4. Darla is the very essence of tragedy: she has been tossed on a sea of events since her birth, and even her final act was at the behest of a greater power. It's truly the saddest story in all of the Buffyverse.
However, there is one bright jewel of happiness that comes to me as I think about her fate. Even though her last day was spent in unnowing service to the causes of another, in death itself she is finally free from control.
Rest in Peace Darla. May you find the peace in death, that you never had in life.