If you think the zoo keeper have used a wrong method to save the kid, what will you do instead?
Imagine the situation, you're the zoo keeper, maybe is on the process to prepare alternative methods to save the kid which without involve killing the gorilla. Before preparation complete, the gorilla lug the kid in the pen a little forcefully, which in some way can be making the situation more panic, what'll you do? Continue with the preparation of previous plan? Tranq the gorilla with hoping the tranq will not anger it which may end up express its anger to the kid?
In that immediate situation you have no time to thoroughly think about it, your panic may make you more easy to misjudge the situation. You haven't thought about the gorilla's intention to lug the kid can be to bring the kid away from the screaming of tourist. It seems like the movement of lugging have make the situation more tension, can you allow the risk that "the tension may increase" and let it happen? After the gorilla lugging the kid in the pen, can you allow the risk "what it may continue to do in a more forcefully way" and let it happen? What'll you do?
How successful the rescue going on will be depends on how experience and wise the zoo keeper is, whether anyone making mistake in that situation (like the tourist screaming make the gorilla lugging the kid which intensify the situation, or misjudgement made by the zoo keeper), the preparation time of the equipment require for the rescue. Note that panic can make the zoo keeper to more easy in making misjudgement.