I am opposed to capital punishment in all situations. Terrorists who kill civilians make me feel close to ambivalent - if Bin Laden or his associates died I would not mourn - but still, on balance, I don't believe the state should kill people. My belief is moral - it is not based on a cost-benefit analysis of the death costs versus deterrance effects. To the contrary I think that potential killers might be deterred by the prospect of judical killings so that capital punishment is probably efficient even if it isn't just.
This New York Times discussion looks at some recent work on the issue.