Here's the thing. Human life is not precious, and, while individual opinions may vary, society as a whole does not treat it so. States, organisations, companies, etc. are perfectly willing to cause death and mayhem against their fellow man when it suits them, either by co-mission or omission. Hell, Obama has killed more kids with drone strikes than any Gorilla, and there are far worse out there. IIRC Madeline Albright responded to the claim that US sanctions against Iraq had resulted in the death of 500, 000 Iraqi children after the 1st Gulf War with some wishy washy rhetoric about American exceptionalism. Governments frequently decide to cut services or enact policies which they know will result in death and suicide. Corporations too, particularly mining companies in Latin America, are quite willing to displace indigenous people, even sending thugs to kill them, poison peoples water supplies and fresh air, and worse. Socially too, we deem some forms of human life as acceptable casualties. Things like alcohol and tobacco, event though we know they cause illness and death, are still legal and socially approved (more so alcohol), often defended under the banner of 'personal freedom'. And I am by no means criticising this view, and I essentially hold it myself. To go further, alcohol is so ingrained in many nations that not drinking is considered strange, even viewed as a sign of deviancy. It seems that human life is only precious when it is convenient.
Having said that, I'm not trying to say that the zoo made the wrong choice. To be honest, I don't know if there was a right choice. My problem is with the idea that human life is precious by default, which is not really how people actually treat other people. We don't have any categorical loyalty to our own kind. A potential friend is also a potential enemy. Not that I think this means humans are inherently cruel to each other, just that we come to understand other people via relationships, not categorical loyalty. For example, if an adult fell in, the answer would be obvious, because we expect an adult to both look after themselves and to know better. In fact, we rely on adults to do this, or society would have problems. But for a kid, especially a young one? The zoo also has a duty of care to its patrons, do it had to do something. Perhaps shooting the gorilla should have been the second option, rather than the first. Sure, this could have resulted in the kids death, but here's the really irony. Sure, many are upset over the gorilla's death, but in 6 months time I doubt anyone will remember. If the kid was killed the only difference is that the kids family would be in mourning for a long period. The rest of us, would forget about it just like we will about the gorilla. Personal tragedy is just not that important.