To definitively prove that the DP deters/doesn't deter murders, we would have to set up parallel universes where all conditions are the same except for the death penalty. You note above that California has the death penalty, but murders are increasing. But the question is, would they have increased more if the death penalty were not in force?
Which is why the deterrant argument is faulty by nature. It's speculative. Here we are, some 40 years since the death penalty was re-instated, and there's no real way to measure whether the DP deters. If the ultimate penalty does not demonstrably deliver the ultimate benefit of reducing crime in a significant way, why do we give power to the state to take lives? The only reasonable impetus, therefore, is vengeance, which, spiritually speaking, is contrary to the teaching of Christ.
From even a non-spiritual perspective, we naturally want an eye-for-an-eye, but in the satisfying of our vengeance, we give the state power it should never have, except in extreme circumstances. We all cringe at the thought of the government tapping our phones, but we're willing to give the government the power to kill us? It does not compute, and under the wrong circumstances (say, a radical secularist government and society that seeks to banish Christianity - yeah, unlikely now, but talk to me in twenty years) such power could be abused to an extreme.
Don't get me wrong. I'll be the first to admit that I get a certain, immediate feeling of satisfaction when I hear that a violent killer has been sentenced to death. But in grasping the bigger picture, I have a severe conflict with the DP. I'm not sitting in judgment of anyone who supports the DP. I'm working out what the Church is saying versus my natural impulse. This debate has been a good one.
bump.
I'm not saying the deterrant arg is faulty by nature; I'm saying it can't be definitively settled by statistical swings. That data will figure into the argument, it's just not definitive by itself.
Or maybe you could look at it like this: the only reasonable impetus, therefore, is justice. Is justice a legitimate goal for us to try to achieve by public means? The question is not without difficulty but I would answer yes.
It is important to bear in mind that no work of man will be perfect and therefore perfect attainment of justice but whatever means is impossible. My feeling is though that American justice is very far from perfect. The guilty are freed and the innocent punished all too often. But also the punishments meted out to the guilty are not fitting either too severe or too lenient.
My theory of government is that we have it because we've learned in the long course of human history, and in some part by our evolutionary heritage, that we're better off with it than without. IOW it improves our lives. It's a short step from that view to my moral view of government that, in order to fulfill its role, it is obliged to try to make us better off always and never worse. Further I think that it is obliged to seek continually to improve.
Who should be responsible? I think the members of our justice system, lawers, judges, legislators, police, have the primary moral responsibility to continually refine and improve it. I think they're not fulfilling that responsibility.