I hope y'all will forgive this long post, but there are a number of points I wish to clarify before addressing "Morality".
First of all, the tactical situation plays an important part in decision making. As a reminder, to justify lethal force one must have at least a reasonable belief that their life is in immediate danger from one or more others.
Thus, WildAlaska's ad absurdum examples of 9 year-olds, drunks and poverty-stricken children committing misdemeanor thefts are not applicable. Nor are we talking minor trespassing on your property or other petty crimes.
"Retreat" differs for each individual too. Someone like PlayboyPenguin probably has far more options in retreat than I do with bad knees and either of us has more options than my visually-impaired 87 y/o mother.
If the situation is one where I have a legal right to be where I am and I am confronted by someone intent on armed robbery (knife, gun, steel pipe, chain, etc.) or who, by words, gestures or actions makes it clear he intends to either kill me or do great bodily harm, then lethal force is but one answer (albeit an effective one) to the situation.
We are all endowed with a modicum of brainpower and that must be used to size up the situation based on the facts as known at the moment and any amount of that uncommon common-sense we possess.
In a situation where one is confronted in the parking lot of a grocery store by a knife wielding subject who demands not money, but the bags with bread, fruits and meat he may be attempting to feed himself or family. Jumping inside the car and locking the doors to avoid shooting him may be the better decision.
"Retreat in complete safety" is something of a question-begging term. What is "complete safety"? There is an online video I've seen (and can't find now) where a woman retreated into a busy market to avoid an assailant. It shows her enter and approach several employees a moment before the thug runs in and beats her to the floor with fists and feet. No one in the store does anything effective to stop him.
Let us also be clear on one thing - this discussion of so-called "proactive killing" goes beyond self-defense. When defending myself, I will use all the skills I can muster, recall the training and practice I've had in order to stop this person from seriously injuring me or killing me. If lethal force is legally justifiable then the employment of that force isn't a question. The question is whether lethal force was gratuitiously applied.
Defending one's self with the goal of killing your opponent goes beyond self-defense. The goal is to cause the threat to cease as quickly as possible. Since lethal force may legally be used, I'm not overly concerned about the perpetrator's survival, but my own. If he survives his wounds that's an acceptable outcome. If he does not survive, then he instigated his own demise by committing a violent act.
The Morality or Ethical Question
I'll be blunt when it comes to being in one's own home or similar place (e.g. hotel room) and someone unlawfully enters the place with a criminal intent. There is, I believe, a moral duty to enforce the concept that your domicile is an inviolable sanctuary from the public and especially from those with a criminal or violent nature. Even Biblical passages support the notion that a "thief in the night" may be killed by the resident of a home without it being a proscribed murder. You can work out why there is a moral duty to enforce this concept on your own, I am sure.
In a working environment or public venue, there are more variables and questions that must be addressed. And I believe that the tactical situation will also aid in dictating whether retreat is a moral decision or not.
Supposing someone with a knife or club attempts to carjack your vehicle at a 4-way stop sign. Given the choice between shooting and simply driving off, the latter choice may be the moral one. He does not have the capability of injuring you as you put distance between you.
Likewise, if you are in a business and near the east exit when a man near the north entrance begins to rob the business, slipping out the east door to call 911, be a witness or to protect yourself may be the most prudent thing to do.
The morality question becomes murky when we introduce an actor like James Huberty who committed the San Ysidro, Ca. McDonald's shootings. You're near the east exit and the first indication of danger is hearing gunshots. The source is a man who has entered the north entrance and is shooting anyone nearby. Even as you assess your options, bodies are hitting the floor.
In this case, WA's question of a "moral duty to retreat" must be weighed against the moral duty to stop multiple murders. Or, to put it another way, is it moral to allow innocent people to be slaughtered so you may escape, when you have the means to stop the slaughter?
Self-preservation is a natural instinct and is legally permitted. But morally, each person has to answer the question of whether they can live with their decision. This is not to suggest some foolhardy attempt to be a hero or that getting out of the killing zone is an act of cowardice. Tactical considerations aside, if one is well armed and elects to flee when ample opportunity to end the tragedy exists, then that person has to live with the moral repercussions of allowing innocents to die for their safety.
We can also argue whether a violent criminal, by their own actions, have declared contempt for the laws and morals of civilized society and deserve every single legal and moral consideration in how that society treats them.
As for me, I don't see a moral or ethical duty to retreat except in very limited circumstances.
