It is not natural for alligators to prey on humans and it is very rare.
That's not really true.
Fatal attacks are rare but alligator attack humans occur on average a dozen or more times a year in both Florida and Louisiana.
They're opportunistic predators, and while they don't deliberately stalk and hunt humans it's pretty well known that when you enter the alligators' natural habitat there's a fair to middlin' chance that if you replicate alligator prey behavior (like coming down to the waters edge for what appears, to the alligator, to be a quick drink) an alligator might get it in his head to treat you like prey.
it's pretty much common sense.
I've fished all over Florida and you always need to be mindful of possible alligator or snake presence when fishing fresh water.
Those that do must be destroyed because they have lost their natural fear of humans and will attack again.
That's not necessarially true in this case.
It isn't as though the alligator stalked a six foot tall, two hundred pound man and made a deliberate attack.
It's a case of a tiny little kid blundering in to the alligator's habitat and doing, as I said above, exactly the kind of thing that alligators look for in their prey.
The gorilla is a entirely different story as they are far more intelligent relatives of humans and are an endangered species. That killing could have been avoided.
That is idiotic nonsense.
You're talking about a small boy literally in the hands of a full grown silverback gorilla.
The fact that the gorilla hadn't hurt or killed the boy at the time he was shot is no indication at all that he wouldn't have shortly, especially if the zoo had done something stupid like shot him with a tranquilizer, which would have startled and injured him and would not have had an immediate drop-dead effect.
Gorillas, as you say, are "intelligent relatives of humans" in relation to, say, alligators, but they're not exactly a species that can be reasoned with, eg., "Hey there gorilla, why don't you just go ahead and give that boy back? We're practically neighbors on the cladistic tree you know!"