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Short answer: maybe nothing. Just because you "hate" two different characters doesn't mean you hate them for the same reason. Empathizing with a character, or considering them nonredeemable, is a...
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Source: https://writers.stackexchange.com/a/48197 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
Short answer: maybe nothing. Just because you "hate" two different characters doesn't mean you hate them for the same reason. Empathizing with a character, or considering them nonredeemable, is an extremely personal decision. Many times it will have more to do with a reader's experiences and values than with the character's actions. For most, rape is considered a nonredeemable. Sadism and torture, child molestation, all "non" for me. Many times it is the character's attitude even more than the end result of their actions. I would consider an alien creature who destroys a plant full of humans quite redeemable, if only it could be brought to see humans as "people". If it destroys a planet full of its own kind, less so. And a human who destroys a planet, worse still. But then we have Darth Vader. He was redeemed at the end. Here's the pivotal question. Can you convince readers that the person or persons in question are capable of truly changing? If so, they can be redeemed, _unless_ the reader finds the crime so personally offensive that even the idea of redemption for that person is unthinkable. I don't think you can find a rule that will answer your question. Personally, I'd rely on critiquers who can tell you how they would answer "can this person be redeemed in your eyes?" and if not, what can you change in your story so that the crime is not so completely nonredeemable? Reverse that to make a character nonredeemable. Is it bad enough, if not, what can I do to make it worse? If you get a wide enough sampling from different types of readers you can find your "sweet spot" for villainy. Another aspect of this is how personal your are prepared to make it. Having your POV character hear about a guy who raped a child is one thing, watching it happen is another. And if it is the POV's child... But it's a balancing act. If your POV's experience is too terrible, readers will not be able to tolerate it and will throw down the book. Again, it is very dependent on the personal experiences of the readers.