Fiona is Jonas’s friend. By all accounts she has a big heart and is conscientious and intelligent. She is chosen to be Caretaker of the Old because she often volunteers at the House of the Old before turning twelve and has a knack for caring for the elderly there. She is thrilled with her appointment.
It is not until later that we find out more about Fiona. We learn she has red hair, which bothers the community. Red hair is different from what everyone else has. The community works hard to keep everyone the same.
The Giver chuckled, suddenly. "We've never completely mastered Sameness. I suppose the genetic scientists are still hard at work trying to work the kinks out. Hair like Fiona's must drive them crazy" (Chapter 12).
Other than her red hair, Fiona fits into a utopian society very well. She believes her community is a utopia. From most community members' perspectives, everyone is happy and everything is perfect.
The society in The Giver is actually dystopian. Jonas learns about this later when he finds out what release really means when he sees his father kill a newborn baby. It is at that point that he remembers Fiona is a Caretaker of the Old, and the Old are routinely released. He finds it difficult to imagine Fiona knows about this, but The Giver tells him she is.
"Fiona is already being trained in the fine art of release," The Giver told him. "She's very efficient at her work, your red-haired friend. Feelings are not part of the life she's learned" (Chapter 20).
Since Jonas does have feelings, and has experienced memories of death, it is difficult for him to imagine his friend killing old people, but this is part of her reality. As The Giver explains, Fiona doesn’t know better. No one but Jonas and The Giver actually understand or feel anything about death. This behavior in the community is what makes it a dystopia.
This does not make Fiona a bad person, though. If she were in our society, I sincerely doubt she would be going around murdering old people. She is a good person, and if she lived in a society where it was not okay to kill people, she would not be doing it. If Fiona lived in our world, she would not be a killer.
The author uses Fiona and Jonas's father to create shock and understanding in the reader and reinforce the concept of dystopia. Jonas's father and Fiona have no idea they are murderers. This makes the reader realize Jonas's society is very different from ours, as its members routinely practice what our society considers immoral behavior.