I should've been more specific, sorry. The "right to roam" is mainly practiced in the UK but the same ideas are in the United States and district of Colombia.
http://www.alleycat.org/page.aspx?pid=302
It is a crime to kill a cat in all 50 states
It's kind of a shady area and not a lot of information is available for it for one reason or another. Cats were considered an invasive species because of being brought over by the Europeans. I do
not agree with the law and I think that cats should be treated like dogs when worrying or killing livestock and be shot with no repercussions. But I have read and have had people tell me that there are laws against it. I posted it because I dot want the OP to get into legal trouble for something that was suggested.
There are animals that cannot be killed on your property even if they are harming your animals, unfortunately. Some of those animals (in my state) include; birds of prey, owls, fox, cats and opossums. I looked this up on the DNR website a few weeks ago because I was curious. Farmers can get special permits to kill some of those animals but not all of them. Some of the farmers here have permits to kill sandhill cranes even though they are technically protected. There's a lot of grey areas and special circumstances for where you live and what the conditions are and I didn't even know half of this until I started digging. I always thought it was okay to shoot a cat on my property until someone told me it was illegal and I found out it was. I'm not trying to be *that person* but I'm just putting that information out there. Again, I want to reiterate that I don't agree with it, but that's just what I know. And I may be wrong but I wouldn't want the OP to get into legal trouble over taking advice and doing something as menial as shooting a cat attacking the chickens.
An article about free roaming cats and how to handle them
http://www.mchumane.org/documents/Free-RoamingCats.pdf
Edit: after further reading, opossums are not on that list. I read it in a brochure upstate, but I think I was mistaken.