A cat remains the property of it's owner until such time as the owner relinquishes ownership, or the cat becomes feral. There is no legal precident that I'm aware of to confirm what 'feral' actually means, so great care should be taken to ensure that you are squeaky clean before taking action to destroy a feral cat. 'Best Practice' which is what the courts use when making this kind of judgement requires that the animal is caught alive and scanned by a vet, and if ownership cannot be proved, the animal can then be humanely destroyed.
Interestingly, there is an article in this edition of '