' The Son of the Turtle-Spirit'

This is the first piece of literature we are required to read by our lecturer. After reading it, I am attracted by the maiden's character. She is a clever mother who is able to help her son to succeed. Also, she shows gratitude to the man she loves. Despite knowing the real identity of her 'husband', she still keeps his bones with her and does not throw it away. It is an interesting story based on Chinese culture, yet there's something that I'm not clear with: Since the turtle can change his form into a man, why is that he doesn't help himself to escape from being killed by the maiden's father??