So many focus on that one little word---"innocent"---when trying to draw the false distinction as to why they are so opposed to killing the unborn yet quite comfortable in killing some who are guilty (and committing state-sponsored murder at that!)
Ah, linguistic gymnastics. Next thing you know, these folks will be trying to convince you and me that a horse chestnut is really a chestnut horse.
I'll side with Pope John Paul II on this one. Murder is murder, let God Himself be the ultimate judge of who lives and who dies (save for self-defense and military conflict, which is in essence self-defense).
Life without parole is dying a thousand little deaths each day for decades anyway, and it is something that can be done that does not degrade the moral health of the country in the process.
So if a person kills someone in self-defense, aren't they essentially acting as God by taking the life of their assailant?
Fine.
I'll side with the Council of Trent, Pope St. Pius V (among others), the Catechism of the Catholic Church, the Scriptures and St. Thomas Aquinas.