Let me get this out of the way. I love animals. In any choice of saving an animal or a human, the human wins. Sorry, EOS.
Shooting that gorilla sucks, but if the zoo keepers, the folks that probably love animals more than you or I, thought the little kid's life was in danger, then they did the right thing. It does not matter how the child got in the pen. It was either/or.
Those twitter idiots proclaiming an animal is equal to a human are pathetic. Show me an animal that has built a car. Show me an animal that can set a broken leg. Show me an animal that invented antibiotics or band aids. What animal cultivates to grow his food source or has domesticated other animals. When the "Planet of the Apes" starts and the chimps have guns, we will will talk.
Until then, I say a four year old is more important than even the last gorilla on Earth. Yes, your dog or cat may be more precious than some of the human scum that walks the planet. I don't doubt it. That changes nothing. A human life is far more important than any animal.