Oh, I agree that a cat CAN and sometimes WILL kill a chicken. Some have very strong prey drive and even if they're well fed, they'll try. Even if it doesn't kill a chicken, a cat like that can terrorize them and/or injure them to the point that the chooks are in danger of trauma, injury, infection, illness and yes, death.
Most cats ARE lazy though and even though ferals often present all the traits that make cats dangerous to chickens, if they're fed and given a safe place, they can learn to coexist with chooks.
I agree that live traps and at the very least, trips to the local pound are in order in the OPs case. It's true that if they're found to be truly feral beyond hope, they'll be humanely killed, but at least they have a possiblity of being adopted out too. (or retrieved by their owners, hopefully to be kept safely indoors afterwards.)
I actually think (and this IS a point of argument to some) but I think there is a place for outdoor cats in a barn/coop/farm situation. Cats have served a valuable position as vermine hunters for centuries. If they get immunized, spayed/neutered, cared for etc, I see nothing wrong with having them out there keeping rat and mouse populations controlled. Yes, they'll hunt songbirds too, and sometimes we need to take measures to protect chicks and ducklings and so on.... but, I maintain that they are worth having around within reason. They really like barn mice and rats best because chickens are just too big, and song birds are just too fast. Just MHO (my humble opinion) but I don't expect everyone, or actually anyone to agree with me.