There are a few theories. The first being karma and the mention fo the rain suggests that 'nature' is punishing her. The woman is being punished for her selfishness, her jealousy, her possessiveness of her daughter-in-law, the murders she has committed etc. SHe has 'become'/been bonded to the demon and her real self is obstructed. Upon the removal of the mask where she is disfigured we also she (in visual representation) has been corrupted and consumed by her own wrong-doing. Maybe it is worthy to note that revenge is a big theme in asian cinema.
It could also be assumed The mask is cursed. The Samurai who wore it originally was also disfigured beneath the mask. Many theories suggest Shindo used Onibaba as a metaphor for both communism (peasants rising against the elite/upper classes eg samurais) and the Japanese participation in WW2/Hiroshima etc. The smurai too is corrupted because he is participating in the act of war. The mask symbolises status and power (in terms of social class and miltary rank) and its effect implies that what it stands for is ugly and corrupt - hence the disfigurement.
Shindo also reportedly used the disfigurement as a metaphor for the 'hibakusha' - victims of Hiroshima and based the makeup on their disfigurements which suggests both the mother and the samurai are also 'victims'. Victims of war, victims of abuse of power, victims of the past etc. and will never be the same again.
I also remember something linked with the actual mask itself/the mythology in relation to women who have been wronged, but I've forgotton it.
As for the hole, well its probably just to round off the story due to the fact she chucked so many Samurai's down there, and there she is wearing a samurai mask - falling intothe hole. Irony.
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