Cats usually don't hurt chickens. I have large barn cats that love to go out amongst the chickens to hunt for mice and they've also gone to the neighbor's chickens to do likewise when they had chickens. It's almost a symbiotic relationship - cats and chickens have been together for a long long time. And chickens don't seem to be scared of cats, but they definitely terrify cats. Nothing so funny as a cat surrounded by curious hens. In this neighborhood there are lots of barn cats and lots of chickens.
My chickens are in a very very predator proof coop because we do have a lot of racoons. And coyotes. And foxes. I could easily believe a fox did the damage since I've seen them all over Denver at night. Or a small dog. Some dogs are great around chickens and some just love to kills chickens. My two dogs were wonderful around chickens but my SIL's neighbor's dog killed all of her chickens repeatedly.
But I'd bet it was a fox or raccoon. Both of those critters are very very good at figuring out how to get at what they want.
