I do not belive in spirits, sanctity of human life and similar religious nonsense. That is why I think that killing a human is not much different, philosophically at least, than killing an animal. I am in principle against killing, I would not kill a living thing myself, at least when I do not have to, but then again killing is a part of life. We kill to eat, we kill in the name of science, we kill in self-defence and we kill out of compassion. My wife and I have an agreement that if anyone of us becomes incurably incapacited, the other one will arrange for a trip to Amsterdam and euthanasia (since this nauseatlingly sanctimonious society does not allow a person to choose his/her own death). I know that I will also have to euthanize my cats when they become incurably incapacitated. This is not what I would like to do, but what other choice do I have?
So if one does not object too much to the above listed acts of killing, why would one object to killing a criminal? The distinction between voluntary and involuntary death does not help much here, because (i) most of the killings listed above (except euthanasia) are not consented to by the killee and (ii) it is a two edge sword: a convicted murderer did not respect the will of the person he killed, why then we should respect his?
I guess that it is the most rational position resulting from the categorical imperative: a "rational" norm of behavior is the one that we wish to be universal. Universal abstaining from killing is absurd, whereas accepting killing in some circumstances (self-defence, euthanasia, animals) but not in other (convicted criminals) violates the norm of universality.
Wojtek