Perhaps there will be no need for the death penalty, one day.
It may soon be possible to completely erase specific memories
. It has been successfully done in mice. If we can apply it to people, this seems like a good alternative to the death sentence: we are the sum total of our experiences. Change the experience and we change the sum. If the sum is changed, then so is the person. If we can eliminate the experiences that made someone into a killer, we could manipulate them into being peaceful. Will we eventually be able to replace the erased memory with another?
This is a total Dark City
moment. Hypnotism has been used to create false memories
With hypnotism, you cannot erase a memory. You can only suppress it. The memory is still there so the person is still partly the original sum. However, you CAN add memories. The original "person" is still there and can be regained by eliminating the false memories.
However, if a REAL memory is somehow erased, what would take its place? There is a gap. A life altering experience is now irretrievably gone. With proper psychoactive drugs, maybe even a little HD virtual reality, and hypnotism, you can replace items in the killer's mind that made them into a monster and thus transform them into, say, a passive, peaceful, and helpful gardener. Beautifully, the killer would believe that he was ALWAYS a gardener and absolutely loves making plants grow. Just fill in the memory gaps with a life devoted to peaceful pursuits and you could even turn them into the most devoted Buddhist monk.
It also provides for the ultimate in national security: state secrets can be permanently erased. No more tell all books.
The darkside... (You knew this was coming)
Totalitarian governments (think China, N. Korea, et al) would have a new tool to root out dissidents: a sort of "capture and release" program where the person is transformed into a double agent to turn in traitors or becomes a "model citizen" thus creating disillusionment in the former leader's freedom and democracy movements.
If the technique can be scaled, then you could create fanatically devoted armies.
... and there would be not one single surgical scar to be able to tell what had happened to their mind.