Here's the skinny:
- The Church teaches, as she has always taught, that a legitimate government has the authority to execute a criminal
- The Church teaches, as she has always taught, that the government has the responsibility to use capital punishment rarely, when a criminal's sure threat posed to the common good cannot be otherwise met
- John Paul II and the Catechism teach that such circumstances under which the death penalty may be justly and prudently imposed are so rare as to be nonexistent, for practical purposes
- However, neither JP2 nor the Catechism impose the burden of sin on any governmental officers who impose or enforce a sentence of capital punishment - given that the process is imposed and executed as justly as possible
Mark calls this position "death penalty minimalism" which seems a good enough moniker to me. It is consistent with the philosophical principles of Catholic Social Teaching, and with the ancient tradition of Catholic moral teaching. It is consistent with scripture and with all the saints I know of.
Some go further, calling for an all-out abolition of the death penalty; this is acceptable under Catholic teaching, so long as they do not oppose the State's obligation to defend the common good.
Some argue strongly to retain the legal option of the death penalty; this is acceptable under Catholic teaching, so long as they recognize that capital punishment is a tool that poses dangers as great or greater than those it solves, and must be used with extreme caution.
I don't have strong opinions on the issue myself, except to maintain clarity that Catholic moral teaching does not necessarily map to party policy, and may have a variety of practical implementations.