It is not acceptable to take the law into one’s own hands in order to carry out a homicide, unless you are in imminent danger of being killed yourself or to protect someone else from being killed. The Church accepts that, while definitely not desirable, deadly force is sometimes the only option in emergency situations, but it does not accept it generally.
Now there IS a philosophical argument about how far to take that. Does it only apply to ‘heat of the moment’ situations? Could it be stretched to encompass abortionists who are literally just about to wield the knife (or other surgical instrument)? Are we entitled or obligated to step in to protect a life where we are not wanted? (I.e. the mother of the about-to-be aborted baby wilfully rejecting intervention, a person who is committing suicide or euthanasia in a state/country where such things are legal, etc). Or are we only obliged to render assistance to those who request it? And that begs the question, would an aborted baby have wished us to protect it? Can we know? Can we assume knowledge? Can we take the decision ourselves?
These are the questions that need the wisdom of Solomon and I certainly won’t say that it’s acceptable to go around storming abortion clinics and murdering the people inside them, even if, in my heart of hearts, I am screaming to God that it should be prevented. Does that diminish me in any way, by not ‘taking up arms’?
In most cases, we have to rely on prudential judgement. We have to weigh our own safety with that of the person being attacked. For less immediately mortal crimes, we have to consider what punishment fits the crime.
I don’t believe that any killing can be good, but some can be necessary (a ‘just war’ situation, for example). However, if there is even the slightest alternative to killing, it should be that alternative which we choose first. We must always run the ‘extra mile’ in order to prevent death.
Those guilty of a crime should be given every opportunity to repent. Capital punishment takes away the time which a guilty person can use to consider their crimes. It can mean that a guilty person might go to their deaths unrepentant. It is incumbent upon us as Christians to do everything that we can to help our fellow men to find their way into eternal life. When we take a life as a punishment, we fail because we are usurping God’s prerogative and we risk sending a soul to Hell because that person may not have acknowledged God or turned back to Him.
Taking a life as a punishment or because it’s expedient is simply too much of a risk to take with people’s souls.