They had to, he dragged that child through the water, hearing the mother scream "Mummy's right here baby" just gave me the horrors. It would have taken a second to be crushed/drowned, the gorilla wasn't protecting him like in Jersey he was playing with the boy.
It's a terrible thing for the gorilla to be killed, it did nothing wrong, I can't believe a child could fall into the enclosure but I won't blame the parents.
I've been out to places with 3 DC under 6 and it is totally impossible to watch the older ones every second, if you are toddler wrangling you have an "awareness" of the older ones, have generally assessed the safety of your surroundings for how far they can roam. In the same circumstances I wouldn't have thought for a second they would be able to go from looking at the enclosure to falling in! Why would you??
I tell you what though I think ten minutes is an awfully long time and as a parent someone else would have been handed the others and I'd have been in that enclosure with the little one, assuming an adult could fit where the child fell.