WRT the specific set of circumstances set out in the original post, I would just soak up as much information about the perp as possible & call 911. I am not going to jump in front of a car and I am not going to try to interfere with a driver who is not currently menacing myslef or others. I would not want Mr. Auto Thief to lose control and mow down pedestrians. I value
innocent life more than I value my vehicle.
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As to the broader principle, it is my contention that folks are justified in doing whatever they have to do, including use deadly force, to prevent crimes to their property. The thief is not just stealing your car, TV, or family silver; the thief is stealing that part of your life it took you to earn the money needed to buy or replace your property.
I do not value the lives of those who steal from others. They have made a choice not to value others and bring that judgement upon themselves.
Deterrence is a secondary, though welcome and salutary, effect of such a principle put into practice.
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My actions in a specific case will be determined by the local laws and circumstances. I may feel it is right to use deadly force on a thief, but I am not going to endanger my freedom over a push lawn mower. My lawn mower isn't even worth the time I would spend speaking to lawyers & LEOs even given an open & shut good shoot. I am fortunate in that I can afford such a loss without too much hardship.
I see my unwillingless to use all means (up to & including deadly force) to stop the hypothetical lawn mower thief to be morally wrong on my part. We have sunk to the place where, in many cases, we have to choose the immoral path to steer clear of our government's edicts. I will sacrifice that bit of my pride, honor, and neighborliness* to remain free to support my family.
Texas is pretty good about these issues, though.
Handy said:
You might consider that your camp is for making a very short, uneducated guess about what is going on and deciding to kill over petty theft.
I believe an unarmed and law abiding foreign exchange student in Texas died under similar circumstances. I'm also sure that the fella that shot him made a similar quick assessment and decided that shooting was correct due to trespass that was in progress.
If the foreign exchange student was trespassing, it would be incorrect to claim he was "law abiding," now, wouldn't it? Trespassing being a crime & all.
Oh, and "petty" may be a legal term, but what I may consider a "petty" property loss may very well be what shoves another family over the cliff...and very much worth defending with all means available.
* Neighborly/a service to community in that the lawn mower thief won't be back to steal my neighbor's lawn mower tomorrow night, if I send him to his reward in the next life.