There is a threshold of no return.
Probably one of the biggest ways it’s set in cultural history is Biblical. Pharaoh vs God & the plagues. The background is Pharaoh is a self-proclaimed God, and his people are able to mimic the plagues for the first half of them. Each iteration of Gods playing chicken with each other it had “Pharaoh hardened his heart”, and he chose to take back his loss from the other God and go right back into “I’m God and you bow to my will, now go torment this other’s God’s people”.
But the last plague, where it was playing chicken with the lives of an enslaved people’s firstborns vs Pharaoh’s, it’s not Pharaoh who hardens his heart. It’s God.
And people who aren’t thinking this was a battle of Gods get so upset at the “story of the true Deity” that they talk about how it’s a game of unequals and gloss over what Pharaoh was: deity, owner, ruler, and a many headed hydra: cut him down and the next one takes his place as a God and surpreme lord of his domain. Both in reality and in stories, this is the type of character that can’t be redeemed and has to be crushed.
We had a monster just like that in WWII.
During this summer, a writer of dark romances attached her ML to a known killer, and all hell had broken loose in the writing community because this was too close to redeeming a genuine monster for a fetish.
So, there are irredeemable people, often icons.
The problem then comes in at less standard evils and motivations. For example, any ideology that is touted as a victim by a large swathe of people, hiding predators behind it. Generally, its good to assume that 5% of any population is dangerously abusive, and if laws aren’t in place to net those people and cull them from the population, we’re allowing predators to run rampant in that ideology. You’re going to get this easier in ideas you find to be “your enemy” than the ones you support. The problem is that it’s your job to police your side and not allow ideology to hide the monsters. If you’ve never outed one on your own side but always play the underdog going after the oppositions’ glaring issue…well, your underdog crusade is bullying. Are YOU redeemable? Did YOU come from a place of good intent? Didn’t you think YOU weren’t the bully?
At that point, if you can place yourself right there and face what you’re hiding from theoretically (even if you never meet a real problem), then the plea for saving a bully from themselves is actually a plea to save yourself from your own failures.
At that point, you’re ready to tackle saving a bully. You’re saving someone who thought they were justified, but aren’t. Totally different from a true monster.