My rambling take:
Firstly, as you said, it's between a kid and the gorilla, so kind of a no-brainer.
The only video snippets I saw made it look almost like the gorilla was protecting the boy. That's me anthropomorphizing, but I have to wonder what would have happened if there wasn't a crowd of people all yelling and screaming and agitating the gorilla? Perhaps that would have been a point at which they could have used tranqulizers?
The zoo made the best decision they could given the circumstances. It's funny, I saw some comments about how it, "Took them nearly ten minutes to do anything!" Because they always keep gorilla snipers handy just for these instances (or would that be guerilla snipers?).
I'm not sure the mom is completely blameless.
Lastly, it's between a kid and a gorilla, so kind of a no-brainer. If it were between the gorilla and an adult human who went into the cage on purpose, I'd be rooting for the gorilla.