What struck me was the richness of the culture and theology in the story. It is rare to find a story in fiction that so perfectly elaborates upon a setting and the ancestor-dream worship combination appears to have been well explained. I think all of the character motivations are reasonable; the mother was chosen specifically via her greed/pride so it make sense for those traits of personality to continue to be evident. There are two other elements which justify her behavior. Firstly, the beauty of the carved skull contributes to the immortality after death and legacy of the person. Secondly, I also see an undercurrent in the mothers behaviour, one of guilt and sadness over her husband's approaching death. It may be hidden, but the effect of one's love dying often will lead to extremes. If she were to die first, she would be spared.
I would classify this as one of the foremost works in setting description, with a richness of theology/mythology seldom seen.