Not all who oppose capital punishment are absolutist in their beliefs. Many proffer up some cases in which execution is acceptable. Among these, the most prominent are (in the wake of 9-11) cases of terrorism, mass murder/serial killer, and cop killers.
The case of terrorism is perhaps unique. To begin with, if the terrorist(s) are not a member of the just society in which they engage in terrorist killings, then there is no conflict with a rule stating a just society does not take the lives of its own members. If, however, the locus of the terrorist activity and the terrorist are both situated within the same just society, then it is not dissimilar to the second case of mass murder or serial killing. (I’ll come back to another issue pertaining to terrorism later in this piece.)
The exception carved out for mass killings is based largely on social contract theory. That is, the act is outside of the bounds set by the social contract, and thus this actor has forfeited his citizenship in the just society (I think this can be fit into both an Hobbesian contractarianism as well as the contractualism of Kant, Rawls or Scanlon). This would place him outside of the protection of a rule that holds a just society does not take the lives of its own members.
Our final exception- that of the cop killer- is an argument based more on symbolism and/or emotion. In its symbolic role, it’s a means for politicians, and others, to show that they are “tough on crime.” At the emotional level, it is a visceral reaction to what some perceive as an especial affront to public morality.
But are those valid reasons? Might we not make the argument that our police officers, as our military, are engaged in an activity (protecting us) that inescapably places them in harm’s way? And that these individuals have accepted such potential dangers. Is there a societal interest in offering special punishment to the killer of Officer John Doe as opposed to the punishment meted out to the killer of citizen Jane Doe? If there is, I fail to see it.
Now, back to our terrorist situation. Is there a way to still use our notions of a just society in such a way that terrorist activity, no matter its locus, would be a breach of our social contract? I believe there is, if we expand the concept of a just society to include all humans. Now, making such an expansion would not fit into a contractualist account (as evidence by Nagel’s criticisms). Nevertheless, this thought experiment really underlies most international law with respect to human rights.