I hate to disagree with you, but many animals kill JUST for fun!
Domestic dogs are a number 1 in that category. It is 100% a game to them. I'm sorry if you don't know this.
Cats kill just for fun as well! My friends' cat is well fed and brings him humming birds with their head missing, other song birds it has caught and mice. It never eats a single one. And has been know to stake out a nest and kill every single chick/fledgling! This is one reason domestic cats are wreaking havoc in many places on the wildlife and wildlife experts are trying to fight it.
Here is a link to a very recent thread to prove to you that even though you don't think animals kill for fun, or haven't experienced it, in fact they DO... Mind you this dog was not guarding its own territory as my dogs were with their kills. Not that I think my dogs are above it! Prey drive in MANY domesticated animals is VERY strong.
https://www.backyardchickens.com/threads/neighbors-dog-killed-hen-how-to-stay-on-good-terms.1177675/
Foxes will go into a kill frenzy when they are in a coop and the hens running crazy..especially youngsters. Its instinct. Weasles kill for the fun of doing it. I've lived in rural areas with chickens most of my life...ive seen some interesting behaviors. Once..i had a possum that came every other night and took two chickens at a time. All i would find in the morning was two chicken heads on the ground! It took 20 good sized birds in a month before i caught it in a trap. I was shocked it was a dang possum. It was humongous! It was also lucky...i drove out about 30 miles to a wildlife reserve and released it.
Did you in fact have permission to release the opossum into the wildlife reserve? Or just release it to fend for itself in another animals territory and kill off the wildlife in the reserve? Maybe good intention, bad reality for the animal.. and possibly the animals in the reserve as well as
ILLEGAL if done so without permission! Also, may have been feeding babies, though opossum do carry the young in a pouch because they are marsupial. I'm sure that only last until a certain size.. but I only volunteered at a wildlife rehab center and while I learned a lot, there is MUCH more than that I don't know. But I do know that releasing animals in another location is illegal (in most areas) and possibly detrimental to both the residents and the intruder.
If you had permission, that's great!
Otherwise, I hope you will consider these facts before you face the situation again and choose to do the right thing.
Yep, you bet I'm passionate about wildlife! I respect it in all its' forms as well as know that even people who hunt to get meat naturally and animals on feed lots have it bad.. animals in the wild do NOT in any way have it easy. They must always look for food and often have tick or other parasites as well as being quite stinky. Besides, whether I agree with the laws of the land or not I try to abide by them (even the stupid speed limits in some places and using my taxes to support politicians' BS liberal agendas).
Anyways, were are all friends here sharing out different experiences and opinions as well as solutions.
I pride myself on being a part of the *mostly* wonderful, helpful, caring community of BYC where I have always found a safe place for discussion even hotly debated ones!
Though I have no personal experience with fox or other predators YET, my day is coming as I live in the beautiful PNW surrounded by the redwoods.
