I am against the death penalty for a number of reasons, which I largely failed to articulate very well in another thread recently.
One reason - and to me the weakest of reasons - is the possibility of wrongful execution. However, it is entirely possible to determine someone's guilt beyond all reasonable doubt, and I am still against the death penalty in those cases.
My second reason is a moral one. I don't believe that the state should be in the business of killing people. I think only bad people kill other people, and I want the state to be better than them.
Third, there is a very thin line between justice and revenge. Justice is a good thing, revenge is not.
Fourth, when you are dead there can be no consequences. If you do something bad, you ought to remain alive to experience the consequences of your acts. Death completely absolves you of that.
I oppose the death penalty, but for reasons pretty much opposite yours. To me, the chance of wrongful conviction is the strongest reason not to support capital punishment. Life imprisonment allows for the possibility of at least release and partial compensation. Death does not. And there are too many cases where "beyond reasonable doubt" turned out to be little more than prejudice and withholding of exculpatory evidence. Even if there are completely cut and dried cases; having the option available for those cases means having it as an option for all of them. Removing the option entirely eliminates the chance it will be used wrongly.
The moral argument i can understand; though I don't entirely agree with it. For one thing, it eliminates the possibility of defensive wars by the military, or defense of others by the police, when taken to its logical conclusion.
The Justice vs. Revenge issue seems to be a bit of hair-splitting to me, since it is effectively included in the moral argument.
The "consequences" argument is to my mind a red herring; not to mention directly contradicting one of your earlier arguments against capital punishment. A particularly nasty red herring, since it can be, and often is, easily used to justify torture and other acts of inhumanity. It has certainly been the justification for various bloodthirsty fantasies posted on this board in related threads. It is far too similar to the arguments in favour of "extraordinary rendition". What, exactly, are the consequences, and how do you ensure that the perpetrator experiences them to the extent that you would like them to? Is it the results of conscience? Sociopaths certainly don't have one, and some appear to live quite comfortably in prison. In fact, for some, prison is an improvement to their quality of life; what with getting three meals a day, shelter, and free medical care. What about the mentally ill? What are their consequences? In some cases, the consequence is that they will finally get better management of their disorder; since G-D knows it's extremely difficult for them to get help anywhere else in this country unless they have lots of money. Depending on the prison, the environment can be an astounding brutal and dehumanizing place; to the point where "cruel and unusual" most certainly applies. Morally, a society can be judged not only by how it treats its weak, helpless, and damaged members; but also by how it treats the worst of its members. Invoking "consequences" is just a thinly-veiled justification for revenge IME; and to be morally above that is not to care about "consequences" for the guilty; but only about how to protect the innocent.
I oppose capital punishment, for the sole reason that we as a society can never be truly certain, absent perfect information, that we are executing the true perpetrator of a crime; and even if we do, that said person was and is fully cognizant of the nature of his offense and the consequences of it, and not also a victim of an injury or disorder that damaged his or her ability to make the choice.
No, I tell a lie, I also oppose it on the grounds that one cannot really study a dead person to understand the roots and causes of violence, except via gross anatomy. Understanding why the crimes occurred helps us to avoid or remedy causes of violence.