Ah yes, The Decision. There's a lot that could be said about it. It's one of the most fundamental questions and bears thinking and reflection from time to time. A lot of people take up martial arts in a strange way to avoid making it. They figure that if they're really good they can neutralize attacks without the risk of hurting or killing someone else. Others - you find them in the gun forums a lot - are really keen on the idea of zipping a bad guy. Most of them have never seen violent death up close.
Two very good books on the subject are Lt. Col. Grossman's
On Killing: The Psychological Cost of Learning to Kill in War and Society and Massad Ayoob's
In the Gravest Extreme . Ayoob is a little handgun-specific, but the essential material is very good for anyone looking at these issues.
I would strongly urge anyone with an interest in these matters and particularly anyone who carries a weapon to take Mr. Ayoob's class LFI-1 "Judicious Use of Deadly Force". You'll get a superb twenty hours of the legal and ethical parameters of the use of force and deadly force. Some of it is emotionally difficult. I had a couple sleepless nights. It is solid gold. Lawyers in attendance said the treatment of the legal issues was better than what they got in law school. The second half of the course is twenty hours of world-class instruction in practical pistolcraft. One does not have to take the shooting portion of the program.
I've never killed anyone and fervently hope not to. But I have drawn weapons when I thought my life was in danger and to protect others, and I believe that I would have been able to do what I had to if the situation hadn't suddenly improved. Would it have been moral? Yes. My life is worth infinitely more to me than that of someone who criminally puts it in jeopardy. By extension, the criminal who puts the innocent at risk forfeits the compassion and consideration to which people are entitled. Would it be something I would enjoy and walk away from unscathed? Certainly not. If you have a functioning forebrain and conscience the act of taking a life is significant and traumatic.
To make a somewhat stronger statement I deeply believe that there are times when it would be immoral to the point of depravity not to take a human life. If our spouses, children, guests or others under our protection are under threat of death or terrible injury including sexual assault, violent robbery, kidnapping or arson and one can only prevent it by killing the evil doer it is morally required to pull the trigger or swing the sword. Anything else is an abdication of the most fundamental moral imperative. Any parent, wife or husband who would fail to do so for whatever personal reason is the worst sort of scum and could take lessons in moral philosophy from any cat with kittens and things with too many legs that guard their brood at the bottom of ponds.
Some of this is personal. Some is from my religion(s). "We have been told in the Torah 'If someone comes to kill you rise up and kill him first.' " or
"Fight in the cause of God those who fight you, but do not transgress limits; for God loves not transgressors. And slay them wherever ye catch them, and turn them out from where they have turned you out; for tumult and oppression are worse than slaughter; ... But if they cease, God is Oft-forgiving, Most Merciful. And fight them on until there is no more tumult or oppression, and there prevails justice and faith in God; but if they cease, let there be no hostility except to those who practise oppression" (2:190-193).