This same thing happened some years ago, in Britain IIRC. The gorilla involved tended to the boy, even guarding it from the curiosity of other gorillas in the habit. All zoos keep a rifle on hand for emergencies, always have, but in that case it wasn't used because the gorilla wasn't behaving in a way that threatened the child.
Cut to this current incident, and there is a clear difference in body language from that gorilla from the beginning. You can see he was irritated by the crying, and his rough handling (later throwing!) of the kid clearly showed he didn't see the boy as a harmless, hairless infant - more like a toy, or maybe a new guy who didn't get the memo about who's boss in this enclosure.
I hate to say it, but the zoo did the right thing.
I don't blame the gorilla for acting like a gorilla. I do blame unaware parents and the zoos. If you can design an enclosure to contain powerful and intelligent natural climbers, why the fuck is it so hard to develop a barrier that kids can't squeeze through or hop over?