There's an interesting, and topical, piece by Christopher Hitchens on capital punishment here, with particular reference to its use in the US. Amongst the points that make it interesting is Hitch's acknowledgement of the force of the retributive impulse in people's attitudes to punishment, before he gives moral precedence - rightly - to the primary necessity of not risking executing the innocent.
The link to the Hitchens column comes via Michael Ezra at Harry's Place. Michael also recommends it. However, he finishes with this quotation from Beccaria, which he appears to regard as a clincher:
The argument contained in that question, so far from being a knockdown one, is poor. As I wrote not long ago, responding to a post of Jack of Kent's, if it worked against capital punishment, it would work against all punishment, since the latter generally inflicts on some putative culprit what it is wrong to do to people who aren't culprits.Is it not absurd, that the laws, which detest and punish homicide, should, in order to prevent murder, publicly commit murder themselves?
The implicit argument Michael endorses through the Beccaria quote might have something like this form: if, in doing X, Alice committed a wrong, then it is also wrong to do X to Alice. But that isn't true for all values of X. Thus: X is the taking from a person of stuff in their physical possession and without their agreement; Alice has done just this to Barry who legitimately owned the relevant items; now someone with the proper moral authority to do so confiscates the items from Alice. In the first case X is theft; in the second it isn't. The background circumstances can make two apparently similar acts morally different.
Killing the guilty (as in legal punishment) doesn't have the identical moral significance that killing the innocent (as in crime) does - though the former is also wrong. Indeed, it is precisely the fact that legalized execution risks sometimes killing the innocent that is one of the most powerful points against it. It is because the guilt of the convicted is essential to the justification of executing him or her that it matters so much that there's a possibility he or she might be innocent. But the fact that legalized execution runs this terrible risk doesn't show that the law must never exact penalties which, when exacted by individuals from one another, would be criminal. (Thanks: E.)