Then where does Bundy fit in - a psychopath with no human sensibilities? What about Dahmer? Lanza?
Basically what I read is not being able to understand that killing is wrong makes you blameless for the killing. I've brought up several examples of individual murderers who - because of brain issues - have no idea what is right or wrong. Does this absolve them - make them blameless?
Psychologically speaking a lot psychopaths absolutely know about right and wrong but are compelled by a variety of factors to kill anyway. In fact some of them eventually
want to be caught by the authorities so they can be stipped/fixed or whatever, but they don't have the will to stop themselves- its a compunction.
Society stops them by either imprisoning them or executing them dependant on the level of culpability that they are judged to have about their actions. Some psychopaths are executed by the state (in whatever country thet reside- or prison if the death penalty is not available) whilst others are put in a mental institute and treated. Some are even cured.
On the other hand Buffy stakes all vampires unless they are soulled/chipped. Nothing else matters about a vampires level of compunction- they are deemed simply as 'evil'. That's fine, that's her job.
So, to take your hypothetical and compare it to a real life 19th century case (Australia I think)- there is a man, a killer, who has no empathy- no moral sense of right or wrong. He has a mental disorder (classified as delusion at the time) but then in the course of his crimes he suffers a serious brain injury (head impaled with a rail road spike), survives and is a completely changed individual due to the damage to the frontal lobe.
He is now non-violent and, although he remembers his past actions, has no desire to repeat any of that.
Now the man still went to jail but served a reduced sentence because he judged to be less of a threat/ to blame (psychlogy was in its infancy as was any understanding of how the brain worked- there was a great debate at the time about whether this man was faking or whatever. He wasn't as it turned out)- and I would agree with that.
Jail is not only punishment for past crimes but also a way to prevent future crimes from that same person. But if there was some way to be assured that the criminal has been 'reformed' and will never again repeat past sins then yes, they are absolved to an extent.
The past cannot be washed clean. Those who were tortured and murdered are still dead, and will always be dead. There is no way that can be undone. For me it's immoral to say everyone is blameless. Was what happened to Dusilla simply an accident? Or does it no longer count against Angel, even though Dru is still insane and will always be so? It seems a very convenient world where one can chose to become a vampire, and then be rendered completely blameless because he know understands what he did. It's as if the sudden understanding wipes all the horror clean.
As I remember Sirk was lying through his teeth about that particular translation- Shanshu is basically 'you atone enough and you get to become human'. The demon is cleansed from you (you're no longer a vampire) and the soul part of you gets a second chance at life. That's how Wesley translated it.
The point of Shanshu is not 'washed clean' but to do enough good to balance those scales (that whole Shanshu ressurection is easily one of the weaker parts of S5) and to make amends.
But again that is pretty pointless- lives can't be bartered like that. The dead are still dead and there's no replacing them. All anyone can do is strive to do as much good with the power they have.
Ultimately that's the point of AtS- fighting for the sake of it with no hope of some fluffy happy ending- because that's not on the cards for a lot of the protagonists.