This character experiences negative changes to their morals.

For example, they may begin the story as a kind character but turn cruel.

This is the opposite of moral character development.

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This is literally the trope called Face Heel Turn. The problem is, the moral changes are hard to quantify and humans are not automatons. Unless they are pure omnicidal villains, they are pretty much always somewhere in the area of grey morality and different readers might differently interpret their character changes.

I have read enough of morality threads on the internet, where the same character of a book is interpreted by some to have journeyed from a hero to villain, while others insist the journey was from villain to hero.

A thought that vndb users would be given an ability to judge a sliding morality scale of characters is a terrifying one. That way edit wars lie.