The theme of the arc she's from is about courage. It's hinted at near the start that something is up with her, and it's later revealed that she and members of her guild are all terminally ill at some point (AIDS in her case). The author basically has her deal with the fact that she's dying by showing her becoming courageous and trying to live what's left of her life as best she can, meanwhile she imparts a sense of courage to Asuna (the main protagonist of the arc) as well.
She is a good character but her death was part of the point. When she's on her deathbed she can say she didn't have any regrets, but she was also objectively stronger than Kirito so maybe she was too strong to keep around too.