You haven't been reading the Unabombers Manifesto again have you?
That's certainly the most logically extreme and rare case. And still: Just a person. Whose behaviors happen to be horrifying. People like that are never worth trusting. And always worth punishing (because societal standards are essential).
But they are also a somewhat predictable product of their environment (whether deterministically or probabilistically):
For this, imagine a hundred or ten-thousand people who grew up in loving families in healthy communities in a thriving era, never abused, and even add that none of them are ever taken in by manipulative role models with hateful things to say. Now, watch as they grow up. How many of them turn out totally normal/healthy (in that meh/fine/good/great range)? ...
Now, take those people you imagined and put them all in shitty families, in damaged communities, in an era of fear or deprivation, and perhaps even with dangerous role models, and watch how they turn out. The same people, born with the same human potential, same brains: Except this time the same cohort is worse off. It's predictable that they'd finish less normal/healthy (fewer of them in that meh/fine/good/great range).
Of course, I have to admit I don't know how large the cohort needs to be before horrifying behaviors in the second group start leading to, well, murder. But it's certainly more likely for them than for group #1.
Again. Still horrifying. Still essential to punish. But even in this rare and extreme situation, I believe it's still behavior. Not an inherent thing about the person. That many people may be so far gone (prone to bad behavior) that they are realistically beyond saving, but nevertheless no one is technically irredeemable.