If my Dad finds a cat around his house (way out in the bush, Australia) and it doesn't have a collar and looks feral...
IT GETS THE DROWNING TREATMENT. (in a metal barrel! Whoohoo!)
We really respect the native birds and other endangered/rare species that hang out round my parents place. As cats aren't native to Australia, and if it's not a domestic one, I have NO problem killing it, they're a real problem - wiping out helpless native birds etc. Gassing them with the car exhaust is quite effective also. A shotgun blast to the head, while possibly enjoyable, is a waste of a cartridge. Although we try and make sure the cat doesn't suffer too much. If you see these feral cats, they have NO similarity to domestic, tame cats. If they have a collar, they don't get the gas treatment, and we try and find whose cat it is.
Cats in general simply shouldn't be let run wild on a killing spree. If they do, its silly to blame that particular cat, its just doing what any cat wants to do (kill as many things as possible), but I would enact pretty harsh penalties on the owners, because they're the ones that let the cat kill endangered species, and generally have a bad affect on the environment. If the cat is feral, and there are no owners to get sad, i have no problem with it dieing to save the lives of inumerable other animals. As humans, we're kind of biased for favouring one or two species (and helping it out more often than not) over most others, but do cats really deserve special favour from us in all circumstances? I say no.