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.