To paraphrase Robert Heinlein in "Starship Troopers" (a book, for those illiterates who wasted time watching the movie and never bothered to read the source), a man deserted from an army in which anyone could quit at any time, and eventually kidnapped and murdered a child. Since he was a deserter, he was still on the company's roll, and was punished by the Army.
Who promptly whipped him then hung him until he was dead.
The narrator summed it up by saying "of course he was crazy. What difference does that make? If he was cured, the only thing he could do to escape living with the crime he committed was to kill himself. And if he couldn't do that, he was still crazy. So either way he should die. No matter, the purpose of the military is to protect little girls, not the men who murder them."
We shoot mad dogs, don't we? We don't worry about protecting the mad dog from society.
Some crimes deserve punishment by death, regarless of how crazy the perpetrator is.
Take, for example, the Shoe Bomber. Moussaoui's attempted murder has led to the requirement that every air traveler in the United States, if not the world, has to spend five minutes parading through the airport in bare feet. With ten billion or so passengers flying annually, that man has caused the destruction, annually, of fifty billion minutes. Since a single 80 year lifetime is merely forty two million minutes, the Shoe Bomber has caused the annual loss of over 1000 lives.
He should be executed for wasting everyone's time like that. Since his intent was mass murder, and since he's killing the equivalent of 1000 people a year, he should be punished appropriately.
Sometimes I think we're alone. Sometimes I think we're not. In either case, the thought is staggering. ~ R. Buckminster Fuller
Depends on the illness and the crime. Example if it is just a petty crime? I would have a heart but if someone kills their own child? I could care less if they are mentally ill. Kill em.
~Following My Own Flow~
I certainly don't think anyone should get off for committing a crime because of a mental illness, especially an uncurable one. Let's be honest, does it really matter to the victim what caused their killer to kill them? They're still dead. We can tailor treatment for the criminal to help them, assuming they will ever get out of prison, but even cured, they still need to serve their sentence for the crime they committed.