But you don't know that he is guilty absolutely. This needs to be proven in a court of law beyond any reasonable doubt before the man can be confirmed as a criminal. Eyewitness testimony, while useful, is not the end all be all. Eyes and ears can be deceived.
Sure. And?
Moreover it is enshrined in our constitution that every citizen has the right to a fair trial conducted by a jury of their peers. When you decide to take the law into your own hands you are depriving that man of one of the most fundamental rights our great nation possesses. That is how our legal system works. If you don't like it you can move to somewhere else with a legal system that jives better with your moral outlook. Try Mexico.
Defending your property from being stolen is not depriving anyone of his rights.
^
Nope, it has everything to do with it. The 5th Amendment is there to protect the innocent, and it is crucial that the process of divorcing a guilty person from his innocent status is as meticulous as possible. Because you may believe in your heart that someone who "stole" something from you is a criminal and should be punished, immediately, and without trial, but you'd be committing a grave error. People, and their judgement, are fundamentally flawed, and if you think you have a right to murder someone because you
think they've committed a crime, then you are showing a complete and naive disrespect for the rule of law and for the rights of man. This is why "stand your ground" laws, and even self-defense laws are an abrogation of your human rights. They bypass the rights of the innocent for nothing but blind vengeance, and it is utterly irresponsible.
I'm not saying you have a right to punish anyone for theft without trial, I'm saying you have a right to use force, the minimum necessary, whatever that may be, in order to retrieve property that the thief has stolen from you and is trying to escape with. "Think" has nothing to do with it, and neither does "Punish."
You think people who use self-defense should be punished?
You've said that criminals have no rights,
Not quite. I absolutely agree with stuff like the right to not suffer cruel and unusual punishment (Although I think this depends on the crime, a death penalty for theft would be cruel and unusual while a death penalty for murder, in my opinion, would not). Criminals do have a right to a trial (Although admittedly, they haven't been convicted at this point, which is really the only reason for a trial, we don't really know as a legal system whether the person is guilty or not. If we all magically knew who was and wasn't guilty, there would be no need for a trial. But this is of course absurd)
so they can be killed in flagrante if this is required to stop the crime from taking place - where do you draw the line with this?
Not necessarily a crime. Aggression. Some laws are fundamentally wrong because they have no victim (You can't victimize yourself, or even if you could, you'd be a consenting party so it doesn't matter, you can hurt someone with their [non-coerced, but I do wonder how you can coerce yourself] permission). For instance, a pot smoker is a "criminal" but if someone uses force to stop him, THEY, not the pot smoker, are the person who is doing evil in question, and THAT is the person that should be subject to the "Minimum force necessary" rule, not the victimless criminal.
Does it apply to thieves of all descriptions - pickpockets
Yes, if that's actually necessary, although I find it a bit unlikely that this situation would ever actually exist. I can't really imagine a situation where the only way to stop a pickpocket, especially considering the usually public nature of such behavior, is to kill him. Of course, I could imagine lesser force, such as tackling the person, or asking bystanders to help you restrain him, being justified. Obviously it would be necessary to prove in court later on that he was actually the initial aggressor.
and kids who steal sweets from shops
Kids are a bit of a special case since they aren't generally able to consent. Of course, there are varying degrees of this, and we could debate whether the current age of consent is a good one (I certainly think the system that says that certain actions can NEVER be consented too is axiomatically wrong) but if a kid can't consent, he isn't really a criminal either. So your rights wouldn't truly supercede his in that case and so killing him if he isn't a threat to your life would not be acceptable.
Doesit apply to people who drop litter?
What is littering? Who is it a crime against? If its on private property, its a variant of tresspass. Thus, theoretically, if this was the only way to stop the person in question it should be legal to use lethal violence, but its also pretty absurd that such a situation would actually exist in real life. Of course, using lethal violence when a much lower level of violence would "Work" is not the same thing as using lethal violence as the only possible means to defend your negative rights.
See above.
Or people who illegally download music from the internet?
IP is a moral compromise and doesn't strictly fit under the umbrella of "Theft" per say. Any such enforcement of this kind would also involve other uninitiated aggressions (Such as tresspassing) that would not apply in any real case of theft of real property. Since IP is already a moral compromise, it should be illegal to further stack the deck and use lethal violence to defend it.