This is really sad. The clips I've seen show all show the silverback's body language, and he was definitely not acting like he was threatened by the boy. If anything, he hears the screaming and shouting, and perceives THAT as a threat to the boy. Hence the gentle prodding (well, as gentle as a male gorilla would be with his own kind) and sheltering the boy away from onlookers. And the zookeepers know this.
Unfortunately, the zookeepers know that having taken protective custody of the boy, the silverback wouldn't likely give him up or move away without a fight. And so, the silverback dies.
Right decision. But really really sad.
What's worst is that the age of social media, now and into the future, there's zero chance that little boy will not know of his part in the destruction of such a magnificent creature; now, and as he gets older.
This one is on the parents. And I hope the zoo sues them for the cost of any improvements to that enclosure while it exists in order to have more secure separation between people and the remaining gorillas.