Gorillas are Class A animals when it comes to escapes from captivity, and as such they have to be immobilised as quickly as possible as the damage an ape, of that size and strength, can do is enormous. They are among the most gentle and peaceful of apes but are many more time stronger than a human. A frightened, agitated or protective animal will, understandably use its strength to 'protect itself and other family/group members' very much like humans will...... unfortunately for our weaker human body a protective swipe or bite from an adult gorilla could be fatal, even if it was not intended to be.
Unfortunately in some situations tranquilisers may not be deemed fast acting enough to safeguard the human visitors so the only option is to kill the animal. No-one who works with the animals wants to do this, but the fact is they have to put visitor safety first. Most zoos have trained members of staff to carry out this awful task should the need arise.

Direct eye contact with a gorilla is an aggressive act; a protective/agitated gorilla will respond to prolonged direct eye contact. At the zoo where I work, despite signs explaining this, there are some who think it 'fun' to stare the adult male down and get a reaction from him. It annoys me that there are so many people who totally lack respect and compassion for the other species we share this planet with. I'd like to see those responsible for throwing the stones at Bokito, in with him to see how 'brave and bolshy' they are then; unfortuntely you couldn't get away with that idea.

As they were able to immobilise Bokito and return him safely to his enclosure, I hope this means that he will not be put down. However gorillas have keen intelligence and good memories and they will have to do something to ensure he cannot breach his enclosure again, otherwise they may feel if he's likely to do it again, now that he knows he can, it's just too risky to keep him there. They may have to send him to another animal collection rather than put him down. At 11 years old he will have just reached sexual maturity so may not be the alpha male/silverback as the distinctive long canines and silver hair develops from about 12 yrs.
In the wild, lowland gorillas (the species most kept in captivity) have been seen wading through water up to their chests, so it's the electric wire fencing around the enclosure that keeps the captive animals from exploring further afield. The fact that Bokito breached this indicates that he was pushed into extreme action by the stupidity of a few irresponsible kids. He may now feel more stress if he feels vulnerable and of course the experience of screaming panicking people around him would be very unsettling and stressful for him. Gorillas like calm and peace ... as do most of us.
Whilst I feel for the people hurt and frightened by the experience.... Bokito has no-one to explain the situation to him and my wishes go out to him. His keepers will, I'm sure, do everything in their power to settle him and make sure he recovers from this traumatic experience.