Depends Thom18, on do you want to strictly follow biblical / theological teachings, then you have to research the actual original wording and definition on " thou shall not kill " .
Once you understand and properly defined that meaning then the question begs does one accept that teaching or not.
The death penalty is right up there with abortion, you have that fine of line of special circumstances and a choice between bad and worse or worse and worse.
The what if scenerios aren’t really fair because you can give a what if to any situation. There are though lines in the sand, the secular law / can and does declare what is punishable, and why and sets the terms of said punishment for acts against society and the law. That doesnt mean that those in any religion have to like the penalty or approve of it. Here is a what if… What if a Catholic person is in a field of work, in the judicial system to either decide upon issuing the death penalty upon someone, or is the medical person who actually injects the condemned with the poisons, or is the corrections personel or warden who flips the switch on the electric chair.
I think Catholic who put themselves in that line of work that are directly responsible for someones life, has a big burden on their hands and needs to seek a lot of guideance.
More over, what if people in the judicial system all across America actually refused to enforce a death penalty ? What then ? Does the government resort to the military to enforce the execution ? Federal agents ? and what if they refuse too ? Does the law then get changed to abolish the death penalty ? Or does the government impose a threat of imprisonment for not following a lawful order in the job they chose to serve. ?