Would you still kill this person? His previously vicious character has been more or less wiped clean, and he no longer poses a threat to anyone. He doesn't remember committing any crime, and he claims that he could never do such a thing as he is accused of having done.
To my mind, it would seem pointless to kill him. The person whom we wanted dead -- the unrepentant criminal who prided himself in this crime -- is gone forever, and now we effectively have a new human being, with no connection of character or recollection to the crime.
Perhaps what happens in hell is something like this. Consider that Paul writes that our "old man" was crucified with Christ (Rom 6.6), that the old self is to be put off (Eph 4.22; Col 3.9-10), that the old self has died (Col 3.3), and that the old self has to be put to death (Col 3.5). There is a sense of the word death which has to do with what we may call social identity, as opposed to substantial identity. As a substance, as an individual thing, I still exist; but as a person, defined by my character and habits and relations to others and God, I may die and come into a new life. Perhaps what happens to the damned is something like this.
They are punished for a while, which punishment eventually leads to a sort of total identity loss. The sinful person who had defined herself in opposition to God's will in this way is totally destroyed. The blank slate which remains is then led into a new life in God's kingdom, assisted in some way by the different physiology of the resurrection body. Perhaps here is the wiping of every tear (Rev 21.4), the walking of the nations by the light of the heavenly city (21.24), and the leaves of the Tree of Life for the healing of the nations (22.2).
This is an eternal punishment and destruction, since the old identities of sinful persons have been utterly destroyed and done away with forever. Someone might argue that God, in doing this, doesn't respect the free choice of human sinners to construct their own identities in opposition to God. I simply have to grant the point and insist that, as far as I can understand the biblical descriptions of hell, God doesn't seem particularly concerned with upholding individual autonomy. Either you serve God or you are destroyed -- that's the rhetoric of both Old and New Testament. Furthermore, the same objection could be offered against any understanding of hell, whether traditionalist or annihilationist, since God does not let sinners live their own lives in peace but either punishes them forever or else destroys them. God's not much of a Kantian, nor an existentialist. Anyway, hell is hell. I am not persuaded by the attempts of some modern theologians (e.g., Lewis, Walls) to make hell more palatable or less medieval. Fire, worm, weeping, gnashing of teeth, destruction, ruin: these describe a situation in which people seem to be getting precisely what they don't want, and it utterly undoes them.