The interesting question is to where this view is operative. In our scenario, you are in a situation where a threat of grievous bodily harm does exist and you could use a potential level of lethal force to stop it - but you could also retreat.
If the goal is to protect yourself - and retreat is effacious - do you have the moral authority to use potentially lethal force? That's the question.
The self-defense discussions have never overtly taken the position that you should remove dangerous elements as a preventive measure. It is always to protect yourself.
I would submit that by protecting others we are always protecting ourselves. If someone has by their actions placed themselves in the position that they can be legally shot, I'd say they've demonstrated through their actions that they will continue to be a threat in the future. It's pretty well aknowledged that violent offenders don't have sudden changes of heart where they become productive members of society, go to church on sundays, and help little old ladies accross the street (you get the idea).
By retreating when the law is allowing us to defend ourselves and others we're essentially saying "Let somebody else handle it." Now, some of us have better reasons than others for letting somebody else handle it. But that's still what you're doing by retreating in that circumstance.
That having been said, I REALLY don't want to EVER have to shoot ANYONE. Not because of the legal mess it creates, but because I hope I never "take everything a man has or is ever gonna have" to paraphrase Clint Eastwood in Unforgiven.
By extension of prediction - if you had a car accident when you were hit by a drunk driver and you did survive - you know predictively that this driver will do this again - should you have the moral obligation to execute him or her on the spot? Drunks kill lots of folks.
No one is saying that if you need to protect yourself - you shouldn't. But if you can escape - it proactive killing moral?
Ah, but the shooting of the drunk isn't legal, so it becomes a moot point. I would also say drunks seem to rehabilitate a little easier than violent offenders, though I will conceede that it weakens my above argument. Also I'm a little biased since one of my best friends in High school was killed by a drunk driver and I have very ill intentioned thoughts towards the other individual who was driving the car.
As far as proactive killing being moral or not, I suppose that depends on what side of the concept of the death penalty you stand. Obviously in practice it (the death penalty) doesn't work because of the errors that can be made and the amount of money it takes to follow through with, but the theory is sound IMO.