"Why would an ox that had gored a human be put to death?"

I had always assumed it had to do with future behavior. A lion can't find any more gazelles, or he gets too old to run after them any more. To him humans are stinky and nasty but they sure are slow. At this stage in his life, they're beginning to look more and more like dinner every day, until finally he's realizes it's the best dinner around for him. He wanders into the village where there are unsuspecting people, adults and children, not paying too much attention because hey, lions just don't eat humans and we have more important tasks to do anyway.

The lion singles out a reasonably-sized critter, one big enough to satisfy his appetite but not too large to carry. He pounces on the man, breaks his neck, the man goes limp, and the lion carries him off. Wow, that was easy, and yeah, they taste like carnivore, not at all like herbivore, but it's still edible. I know what I'm having for dinner next time.

Similarly, a circus elephant has absolutely had enough of being poked and prodded and generally jerked around. All her life she has been docile and quiet like all the other elephants in the circus. Everyone sets a good example for the other elephants and behaves and does what these humans tell them to. Finally this one snaps and stomps the hell out of her trainer. That's it! Begone! Die! And the trainer does. Wow, that's pretty easy, I got what I wanted, finally. If I keep that up, maybe now they'll stop dragging me around and making me do stupid things for them.

The dog who has attacked a human has learned how easy it is to drag them off or tear their throats out or mangle an arm (yummy, that taste of blood). Not every dog will attack people, but dogs that have attacked people ... have attacked people. Just from their past behavior we know that this dog has a people-attacking capability, therefore we have to assume that the thought of attacking people occurred to this dog at least once, and if the thought occurs to this dog again, he is more likely to act on it than is a dog to whom the thought has never occurred.

These animals are capable of doing the same thing again. It may take a year or three before they do so, but they have had one rewarding experience from the day they got angry and took it out on a human being--which is to say, they learned from experience just how easy it is to rise up against a human and win.

And so the best thing to do with these animals is to kill them, before they kill a human.