Gorilla killed

[Mister Agenda]

I know tranquilizers aren't as fast working in real life as in movies, but unless the Gorilla was actively torturing the kid or something, I don't think lethal force was needed. I also know he could have killed the kid instantly too, but I don't think you should kill something because a bad thing might happen.

The gorilla was dragging the child through the water at high speed in a way likely to kill the child or at least traumatise it, there was no option but to kill the gorilla, unfortunate though that was.

Dragging behavior from a silverback is often a prelude to a full-on intimidation display, super dangerous for any mere human to be that close to.