
Living in a rural area with many abandoned farms, we have a huge problem with feral cats. After a night of losing several adult chickens to an at the time unknown predator, We put a live trap directly outside the door to the coop on the second night. Surely enough, in the morning the trap contained a large and vicious cat who was caught red handed (literally) with the blood of the chickens he had eaten still on his face and paws. These were some larger breeds, including a partridge rock rooster, but I'm sure they were quite defenseless when attacked inside their coop while asleep at night! I have seen many strays still lurking out there, but after this incident I improved the locks on my coop and always keep my chickens in a covered run and locked up at night, and have not lost any more. I'm here as proof to tell you that a cat definetly can kill chickens!
Good luck and be careful.