It was just an unsupervised/off leash animal with a strong prey drive-AKA a large dog-nothing more, nothing less...
Three of my 4 dogs love to chase the birds that come in to pick up fruit from the ground. All different breeds but all share that instinct. Fortunately the birds are smart enough to get away before the dogs get to them.
It’s so critical to make sure medium and larger dogs are secure in their yards unable to do this kind of damage.
I was talking with a lady the other day who moved in with this guy who has 40 acres out in the desert. They have 4 dogs between them. One day they took off like they do (it’s a country dog thing) and when they came back one of them was carrying a burro leg to chew on. I don’t know if they pack killed the burro or found it out there killed by something else. She came into that relationship with a puppy; now it’s grown up and part of that pack it’s there for life. Those dogs would never tolerate a 1/4 acre fenced yard. They are country dogs now.