Why does Mandy kill Nina Terrence in the book The Cry Of the Crow?

Mandy has to kill Nina Terrance because Nina has become too accustomed to the human life. Her mischievous playing starts to attract unwanted attention, but because she's used to living with Mandy, she isn't exactly accepted in the crow society. Furthermore, Nina Terrance is past the age of dispersing, going out and starting anew. Mandy hard decision of setting the crow free or keeping it as a pet has been delayed to the point where Nina Terrance has been caught up in a bad situation. Neither option is safe for Nina Terrance, so Mandy is forced to shoot Nina for her own good.