If they caught it the chickens may have been the ones to kill it. Mine love to catch and eat mice, frogs, small snakes, basically anything that doesn’t eat them first. I feed them any mice I catch in my traps. I’ve seen them peck a large mouse into pieces so they can eat it, though after they have pecked it so it is all broken up many swallow the smaller ones whole. They really like to play keep-away with these too.
Chickens are omnivores. These things are free protein. With the vigor they go after these things you can tell they really enjoy them. That’s just chickens being chickens.
You probably won’t enjoy reading this, but when I trap critters that will eat chickens, I sometimes open the carcass up and toss it in the run for the day before I dispose of what is left before night (I don’t want to encourage other predators to come around). It’s free protein and they love it.