Based on the author's posts at http://forums.mangafox.com/group.php?do=discuss&discussionid=1124&pp=10&page=2
We are of course free to set arbitrary bounds on what is good and evil, and such become necessary to maintain social order (though arguably rather more as what is tolerable to the order and what decrees punishment as an agent of chaos than outright good an evil, but who can say that they can consistently define the latter two?). In which case we clearly see that our idea of 'good' is not as consistent as we may purport- certainly order seems inherently good, but what of circumstances that require minor, formal 'evils' to maintain that overall good order? We cannot naively hope to uphold this contradiction, and criminal law provides us with a reprieve that places burden on no one individual but on the group.
An act does not become an act subject to judgement without intent, and a justice system has but one- ensuring order. There is no one person that intends this, rather it is the culmination of society's wish to protect itself. So any punishment the justice system decrees is a matter of self-defence. Say you killed a man in self-defence- is that the sort of cold-blooded murder we seem to inherently cringe at? I wouldn't say so, and thus I would agree with the death penalty given the circumstances were grave enough (or equivalently, the threat to the survival of the social order pressing enough).
The matter of justice is not about vengeance or retribution, it is about security. For instance, a murderer receiving the death penalty does not mean that those whose lives he stole would come back, but it will ensure that the threat he poses to anyone in the concerned social order is eliminated. Of course, a lifetime of imprisonment may also be sufficient, but what's to say there will be no unforeseen circumstances where this penalty is loosened and the threat looms once more? And why should the social order bear the burden of sustaining a threat to itself? Hence the notion of security which provided the framework for this entire treatment is also upheld by the swift and unapologetic whiplash of justice.