They WILL kill chickens. I lost 2 hens to possums over the years -- heard the ruckus and ran out to find them attacking my chickens (2 separate occasions). One got in through a small hole in the top of the coop (it had pushed the chicken wire to the side and gotten in) and another time one got in early one evening right at dusk before I had shut the coop and killed one of my hens before I could get to her to save her. Both times, I SAW the possums on my hens, ripping them at their throats. I KNOW the possums killed them. Another dug a hole into the coop and was on top of and attacking one of my hens, but I got there in time to get it off of her. Sorry, but they definitely will kill your hens if they can get into the coop. Be sure your coop is airtight, because they will get in however they can to steal eggs or kill chickens. I recently got a guardian dog for my farm and she's awesome at alerting me if a possum is out there at night trying to mess around my coop.