Did the zoo keepers did the right decision?

When Gorilla's get hit with a tranquillizer they tend to 'bug out' before it effects them. Tranquillisers are also not instant. So in theory, you have a Gorilla which is 'bugging out' and a child right next to him. You may be able to guess the rest. It's unfortunate, but it had to be done.

This exactly, I've heard so many go "use tranquillizers!!" and I have to explain to them the effects. Unfortunately, it was realistically the only option.