At the Library of Law and Liberty Dalrymple notes pro-death penalty arguments that seem valid, though he is opposed to the practice, and comments on others’ inability to maintain the same objectivity:
When I have put these arguments informally to people, I have noticed a curious divide. Those who, like me, are against capital punishment declare that they prove nothing—that there remains no evidence of its efficacy. Those who are in favor of it accept the arguments uncritically. In other words, it is not the evidence that determines their view, but their view that determines their reception of the evidence.
This is all the more striking because the efficacy of the death penalty does not decisively decide the argument either for or against it. It would be perfectly logical to accept its being an effective deterrent to murder and yet be opposed to it. After all, applying the death penalty to motorists who break the speed limit would no doubt be highly effective in slowing the traffic, but few people would argue for it. Efficacy is not all.