The argument in the Catholic Church regarding the death penalty has always proceeded on terms that are not that clear to many outsiders. The essence of the argument for invalidating the traditional permission for the death penalty is that something fundamental has changed about modern penal systems, that for the first time ever we can prevent crime and that it is safe to keep heinous criminals alive and thus the death penalty should be abolished.
The counter in Catholic circles has always been that it is simply not true, that nothing fundamental has changed. It has always bothered me that neither side seems eager to tear the covers off what modern prisons are really like and honestly, openly monitor matters. Here's a data point on the pro-death penalty side. Murder spree conducted by sex offenders wearing GPS ankle bracelets.
Can we trust the state to actually keep us safe? That's an open question.