This has been strongly featured on UK news. I'm not an animal expert or a parent, but have formed the following thoughts:
I've listened to several reports and interviews and I've concluded that it's simply a tragic and unfortunate outcome, with several contributing factors, but no immediate blame or definitive liability.
Perhaps the zoo was complacent over its checking and development of security around the enclosures, but surely it's not just the zoo that checks these, there must have been years of checks and inspections by outside parties, who had presumably considered them satisfactory.
Unless something had broken recently and not been spotted.... but as yet, this doesn't seem to be the case.
It's also completely conceivable that a parent could turn their head in one direction for a few seconds while the child miraculously finds his way into the enclosure - as a parent, you can be watching a child constantly, but you can't be constantly holding, restraining or otherwise be in physical contact with that child.
It's no different from them running off in a supermarket if they see something exciting on a shelf in the next aisle - they are still in your sight, but you can't stop them from running.
If this had been in a wild environment, or in a room with an unknown dog for example, then that's different and I'm sure the parents would be much more cautious and diligent, but parents do not expect a child to suddenly find his way into a wild animal enclosure at a zoo!
I fully trust in the zoo's decision to shoot the animal - in the time it took them to reach this decision I'm sure they would have considered alternatives and made an educated decision based on their expert knowledge and on the situation unfolding in front of them.
A tranquiliser would have apparently been (at best) slow and (at worst) ineffective, due to the agitated state of the gorilla. It could have made the situation instantly worse.
I also heard one expert say that the dragging around of the little boy was a 'display' by the gorilla, prompted by the animal's insecurity and confusion over this new situation he found himself in - sudden noise from the crowd, sudden new presence within the enclosure.
I'm gutted for such a wonderful and intelligent creature to have lost its life, but on the flipside the child was rescued from an unpredictable, life-threatening situation that required an almost immediate decision.
Perhaps if the gorilla had been the one at risk, say someone had broken in (poaching, vandal etc) posing a threat to the gorilla, they would have been shot instead of the gorilla!
Whatever the outcome, the finger of blame would have been pointed in one direction or another by many people and organisations who, in most cases, are understandably reacting instinctively and passionately because, regardless of their knowledge, the matters are close to their hearts.
And it's right that we should review all contributory factors so that we can learn from the event and take measures to prevent anything similar happening again.
However, I have steam coming out of my ears when I hear about death threats aimed at the parents, presumably made by people who are ignorant of most things in life.
To a certain extent I also find it very tiring listening to interviews with animal lovers, who are devoted but perhaps misguided, with little knowledge to back up or justify the opinions they are expressing. I am an animal lover as much as the next person, but would never claim to know what the right or wrong thing to do in this situation was - creatures like gorillas and lions, tigers, komodos etc are fascinating beasts whom we should cherish, but also respect - nobody should ever be complacent about how much we know about their behaviour.
I think that's all I have to offer on this matter
Cheers
TTG