Some people, including many libertarians, argue against the principle of "Preemptive Strike". To be just, you should never strike first, they say, for justice is about doing to culprits according to what they've actually done, and not according to speculations about what they could have done. Yeah, right. With such a principle, you should wait for the suicide bomber to kill hundreds and more, before you may punish him? I hope you can put together the bits to which he was the first to be blown. And a woman should wait for the rapist to rape her, before she may react? And then what, for a perfect reciprocal justice, what she gains is the right to rape him in return? How utterly stupid! Sure, Justice never strikes first. Judicial prosecution against innocents is a bad joke; it isn't Justice to kill a menacing criminal, or to put to jail a burglar who failed at his burglary, or a would-be rapist who was arrested before he could have his ways, or a fraud who failed to deceive his target. No it ain't Justice, and those who claim it is are morons or frauds indeed. But there is such thing as Police, and Police is exactly what these things are: preventing villains from doing harm.
Police action against threatening enemies is quite desirable, and it may sometimes include killing. And when the threatening enemies are organized on a large scale, the police operations may themselves be organized on a large scale, too. When is a threat enough to justify force, and what force? Good question. Sometimes, the answer is easy: if someone wields a gun, and you know he'll be trying to shoot you, you better shoot him before he shoots you. Sometimes, the answer is less easy: if he says he'll come in your sleep and slit your throat, and he really means it, and you know you'll otherwise be defenseless, you may be well advised shoot him on the spot, while you still can. Sometimes the answer is quite difficult: if some people from a loose group make general but serious threats, but you can't be sure exactly how far they'll go, and when they'll strike whom, then you're in trouble.
So when the answer is difficult, you may react too little or too much, or against the wrong people, or at the wrong time. If you react too little, the sanction will be your being victim of aggression. If you react too much or otherwise wrong, the sanction will be you'll have to repair your wrongs. And in the latter case, as always, if you can't settle with someone who says you've wronged him, you'll have to face Justice. There is nothing particular about Police actions here, really: when you do wrong, you have to repair your wrongs. Justice and Police are quite independent things, and confusion is only brought by government monopoly. And based on this confusion, the french "justice" system releases dangerous serial rapist-killers who "did their time", whereas it secures for decades peaceful and productive citizens who are not a threat to their fellow citizens, and are prevented from compensating their victims — or worse, who are injustly imprisoned for defending themselves rather than let the State "defend" them.
In any case, in Police operations as in any Human Action, though principles may guide you, the only way to ultimately find out the outcome of an action is to try it out, if you dare — and face the consequences. So you better do the best you can think of. In a free society, you may subscribe an insurance against such reparations and prosecutions; this way, you may share it with other people who are ready to voluntarily share yours (presumably under the condition that you share theirs); but your subscription fee will necessarily increase if you keep being wrong. And so ultimately, you can't evade your responsibility. Tough nuggies. That's life. Blame Nature. Blame God. But you just can't cheat with Reality.
Some people are afraid of taking such risks and facing the consequences. They call for a magic solution to take these risks away from them: Government. But Government won't take away the intrinsic costs, risks and uncertainties in Human Action — it will only concentrate these costs, risks, uncertainties. And then, you find the same people who say they want to be relieved by Government complain that Government doesn't do what it should — go figure! Truth is, Government is but a con game, whereby the mass of citizens bear the costs, risks and uncertainties, whereas those who play the Game of Power and Parasitism will reap the benefits. The risks are still there, it's just not the people who decide who bear them anymore — unaccountability.
As the citizen of some Government, you're probably a victim of this con game. Do you pride yourself as not being a victim? You're probably deluded. If not, well, then, which is worse, you're a criminal; for forcing other people to do your police work, through taxes, laws, draft, etc., is nothing but racket and slavery. And since you're a criminal, don't be surprised the day your victims come for justice, or the day they decide they better take police action against you. Oh, you may deceive a lot of people for a long time; but when you spread stupidity amongst people, you spread something you can't control, and that will ultimately turn against you, too. Tyrants, you can't last eternally; your days are numbered.