The problem is, no statistical movement can settle the case either way. We always have to scan a whole range of arguments and data when trying to come to a conclusion about the death penalty.
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?
Let's look at it from the other side: in my state, Michigan, we don't have the death penalty. Let's say we institute it and murders drop 15%. Ishmac rejoices and says,"You see, I'm right!" But I would be dumb to claim this as definitive proof because murders may have dropped off for some other reason (eg, all the murderous types left MI because the economy here is tanking and there are fewer people here to rob, beat or kill). Maybe they all move to warmer climes like CA,or better hunting grounds like Chicago, where the death penalty deters some from committing capital crimes--we just don't know. We would have to run our parallel universes and see whether the decrease/increase would have been greater/lesser without the death penalty. So a mere increase or decrease in one direction won't make the case for either one of us, even if the stats seem straightforward.
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.