I have yet to see a death case among the dozen coming to the Supreme Court on eve-of-execution stay applications in which the defendant was well represented at trial... People who are well represented at trial do not get the death penalty.
Can the state, which represents the whole of society and has the duty of protecting society, fulfill that duty by lowering itself to the level of the murderer, and treating him as he treated others? The forfeiture of life is too absolute, too irreversible, for one human being to inflict it on another, even when backed by legal process.
Our criminal justice system is fallible. We know it, even though we don't like to admit it. It is fallible despite the best efforts of most within it to do justice. And this fallibility is, at the end of the day, the most compelling, persuasive, and winning argument against a death penalty.
We as the Church need to express wherever appropriate and wherever possible our stance against the death penalty. We need to talk about it. A lot of people don't feel comfortable in doing this but I think we need to, as the Pope says, preach the whole gospel of life.