Play behavior, or play-like behavior, is one people studying animals are always at a loss to explain. Generally they attribute it to training or socialization, but there are always anecdotal accounts of animals doing something “just for the hell of it”.
Not what you were hoping for, but I had also seen a show on TV long ago (maybe National Geographic), in which different groups of chimpanzees had a war with one another. Group A and Group B fought and killed one another for some reason that I can’t remember now (territory?) I was going to post this as a vague example myself, but now I can only add to your vague example.
Another I can think of was the Dalmation we had when I was younger, about 7 or 8. She had a house in the backyard with her food and water dishes on the ground in front of the house. She would lay on her belly looking as if she were sleeping and if a bird landed near either dish, she would swat it, usually killing the bird with one hit. She’d then carry the bird to a remote corner of the yard and resume her position half in and half out of her house and swat the next bird to come along. Now whether she was purposely pretending to be asleep to ambush the birds or whether she just liked to rest that way and got really territorial when a bird landed to close to her food and water is wide open for debate.
Insofar as it is possible to ascribe any motive to animal behaviour then animals sure do kill for pleasure. Several people have mentioned cats killing for reasons other than food. Perhaps the best example of animals killing for sport s dolphins killing porpoise. Dolphins are anatomically unable to eat porpoise and the two species don’t compete for food, so the only plausible reason for the (very slow death) inflicted on porpoises by dolphins is because dolphins get pleasure from killing other species.
I’d be reticent to conclude anything from accounts of foxes and weasels killing poultry. Research has shown that foxes will return and remove and bury all dead birds if given enough time. The reason it appears that foxes kill poultry for fun is because they return and discover another large predator has hijacked their kill and wisely decide not to fight them for it. I can’t speak for weasels but it seems likely they are doing the same thing.
I know of some raccoons that wandered into the back patio/pool area of a bed and breakfast, yanked all the koi out of the koi ponds, mutilated the shit out of them and then took off.
This happens when man…which congregates livestock together for convenience…plays havoc with a predators natural kill instinct.
When a weasel or fox or whatever comes across a chicken, it kills it then eats it.
With many prey congregated together, the predator HAS to kill ALL OF THEM before it starts eating…its just how the instinct is wired into their heads.
How certain are you that the mutilation wasn’t just eating the choicest bits? Lots of animals, even big animals like bears, will selectively eat the best bits, often the fat, of fish when they are available in large quantities.