Sure, we'll buy that Jennet was betrayed. Her sister forcibly took away her only child and then allowed—at least from Jennet's perspective—him to die in a horrible accident. Way to take care of your nephew, Alice. But The Woman in Black complicates the theme of betrayal. Did Alice really betray Jennet, or was she just trying to provide Nathaniel with a better life? If the accident was no one's fault, could it really be called a betrayal? And what kind of bonds of trust were broken when little Joseph died? Did Jennet betray Arthur—or was he simply betrayed by his sense that the world is a reasonable, fair, rational place? Deep thoughts, Shmoopers.
Jennet feels so betrayed by her own family and community for not allowing her to raise her son that she takes it out on the community even in death.
Arthur thinks he's safe and sound after he leaves Crythin Gifford, so he experiences the death of Stella and his son as betrayals of his sense of safety and security.