It is interesting that your arguments against the death penalty as it is in the current system(s), are, as opposed to the scientific litterature I have come across, based on economic and (dare I say it) evolutionary factors.

You base an argument against the death penalty heavily on the negative economic impact it has on both sides, and while not untrue, it does imply that an economic value can be attributed to a life expressed in the form of money. Using your form of argumentation, there should be a cut off point in terms of cost as to how valuable a life is (and thus how "useful" it is to prosecute, or not prosecute). In other words, if the cost is above a certain amount, it would be more beneficial to society to not punish a person despite the high likely hood of said person having wronged society and thus being guilty. The leap from right and wrong, to cost-efficient vs. cost-inefficient, is not long, and a bit risky in my view. It is, as you say, applicable to the issue of the death penalty, but what about other major crimes such as RICO cases? They may cost even more, but should they not be done because of that?

Another interesting point is that you mention how natural it is for us humans to kill. Whether created so by God, or as a result of neccessity, "to kill is human nature" seems to be the message you are giving. In that sense it seems a bit strange to oppose a system of punishing those that wrong us by killing them. That is, after all, only human nature? You equate it with humans playing God, but the death penalty as a modern expression of natural instinct seems an equaly viable argument to me.

All in all, I found it an interesting read. The economic aspect is for me new and interesting, but also a bit frightening in terms of what and where it could lead too.