My husband and 3 year old son both have Lyme's disease (we live near apple orchards so there are deer aplenty). So I will gladly welcome any opossum I see in my yard.
I wouldn't have killed it. They eat ticks, rats, mice and even venomous snakes. While they can be a danger to your chickens, if your run and coop are secured, you should be okay. They also scare pretty easily.