This does not absolve them of responsibility for crimes or harm done to others (In the cat converter discussion, this is the punishment a court would give them if they were prosecuted. Not a citizen executing them.) , but it does grant them the same humanity as everyone else. This is a part of why we don't believe in court ordered execution in Australia.
You conclusion feels very representative of the US on killing each other, clearly the superior option to being a psycho or being the victimized 'decent folks' is to improve the quality of life for the thief and create disincentive for criminality by providing opportunities.
Instead you focus on the retribution/threat element, using your almighty constitutional rights to enforce the protection of a car part. Look, it has been many many many years now in which US citizens have had guns and the right to shoot motherfuckers, how's that violent crime rate? what about thefts? muggings? any less cat converters getting stolen? Funnily enough, yes! Just in others countries that aren't so focused on punitive bullshit that they think its at all ethical to kill anyone for a car part.
If an individual is committing a crime against your property, I do not believe you have the right to kill them. In this thread a lot of people are relying on the logic that stealing your cat converter could lead to you losing your job and spiraling into homelessness.
If someone is doing something that you can reasonably interpret as trying to kill you (or assault you) then you are allowed to defend yourself. This may be shocking to people of your perspective (not a personal attack), but in Australia individuals have been sent to jail for the second punch they have thrown after the first punch had effectively reduced the risk of death or injury to 0 (eg knocked out the criminal, the criminal began fleeing). This is exactly what I am referring to when I said that we grant criminals the same humanity as everybody else. In Australia (and some countries in Europe), Criminals DO expect mercy or pity from their victims, lest the victim risk legal trouble themselves, and if you go and compare the crime rates and violent crime rates of these countries with those that do not practice this philosophy, I think you will find that our harm minimization policy works better.