If the gorilla decided to be violent, or even to protect it in a way that would be fine for a gorilla, it could still brutally harm or kill the kid. The only safe option was probably to prevent any gorilla from getting near the kid. They are simply so much stronger than humans and probably don't know how gentle they have to be. And that's if the gorilla has good intentions. As humans, our job is to protect humans. In a human-for-gorilla scenario, the decision is simple. I would have taken that shot to kill the gorilla. I'd hate to do it, but that's the only way to ensure you can get the kid out alive.  However, I would prefer to try to save the kid in the style of Chris Pratt in Jurassic World. Just go into the cage and try to get the kid without the gorilla harming him. Let the people behind you use the guns in case shit went down.