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Cennekie: People with Authority, especially those from an religious organisation, who have more authority over people than even a government or royalty, always fear those who might have more authority than them, especially those who might be actually selected by a true god.
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TheSchney176: From the looks of it, the Prince was a Closet-Sadist, but that Saintess turned her into a Open-Sadist+Yandere.
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Swift: It was the Saintess who seems to have triggered her Sadist nature to the extreme, so that she would mess-up MC's life and kill him in the end (probably the Saintess didn't want the Hero to exist, because it would lower her own value, along with the value of the religious organisation).
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Zugzwane: If a series like "Redo of Healer" can get it, then this one could also get. But then again, the concept of this series is quite different from "Redo of Healer", as this series doesn't have that much rape or sexual assault plots. And there are lot of other plots, like cannibalism, human-experimentation, exploitation, betrayal, etc. So, there is less chance to cash in on the sexual aspect in this series, unlike "Redo of Healer".