Now Gorillas are considered 'gentle giants' at least when compared with their more aggressive cousins the chimpanzee, but a 400+ pound male in his prime is as strong as roughly 10 adult humans. What can you bench press? OK, now multiply that number by ten. An adult male silverback gorilla has one job, to protect his group. He does this by bluffing or intimidating anything that he feels threatened by.

Gorillas are considered a Class 1 mammal, the most dangerous class of mammals in the animal kingdom, again, merely due to their size and strength. They are grouped in with other apes, tigers, lions, bears, etc.
While working in an AZA accredited zoo with Apes, keepers DO NOT work in contact with them. Meaning they do NOT go in with these animals. There is always a welded mesh barrier between the animal and the humans.
In more recent decades, zoos have begun to redesign enclosures, removing all obvious caging and attempting to create a seamless view of the animals for the visitor to enjoy watching animals in a more natural looking habitat. *this is great until little children begin falling into exhibits* which of course can happen to anyone, especially in a crowded zoo-like setting.

I have watched this video over again, and with the silverback's postering, and tight lips, it's pretty much the stuff of any keeper's nightmares, and I have had MANY while working with them. This job is not for the complacent. Gorillas are kind, curious, and sometimes silly, but they are also very large, very strong animals. I always brought my OCD to work with me. checking and rechecking locks to make sure my animals and I remained separated before entering to clean.

I keep hearing that the Gorilla was trying to protect the boy. I do not find this to be true. Harambe reaches for the boys hands and arms, but only to position the child better for his own displaying purposes. 
Males do very elaborate displays when highly agitated, slamming and dragging things about. Typically they would drag large branches, barrels and heavy weighted balls around to make as much noise as possible. Not in an effort to hurt anyone or anything (usually) but just to intimidate. It was clear to me that he was reacting to the screams coming from the gathering crowd.

Harambe was most likely not going to separate himself from that child without seriously hurting him first (again due to mere size and strength, not malicious intent) Why didn't they use treats? well, they attempted to call them off exhibit (which animals hate), the females in the group came in, but Harambe did not. What better treat for a captive animal than a real live kid! 
They didn't use Tranquilizers for a few reasons, A. Harambe would've taken too long to become immobilized, and could have really injured the child in the process as the drugs used may not work quickly enough depending on the stress of the situation and the dose B. Harambe would've have drowned in the moat if immobilized in the water, and possibly fallen on the boy trapping him and drowning him as well. 
Many zoos have the protocol to call on their expertly trained dart team in the event of an animal escape or in the event that a human is trapped with a dangerous animal. They will evaluate the scene as quickly and as safely as possible, and will make the most informed decision as how they will handle the animal. 
I can't point fingers at anyone in this situation, but we need to really evaluate the safety of the animal enclosures from the visitor side. Not impeding that view is a tough one, but there should be no way that someone can find themselves inside of an animal's exhibit. 
I know one thing for sure, those keepers lost a beautiful, and I mean gorgeous silverback and friend. I feel their loss with them this week. As educators and conservators of endangered species, all we can do is shine a light on the beauty and majesty of these animals in hopes to spark a love and a need to keep them from vanishing from our planet. Child killers, they are not. It's unfortunate for the conservation of the species, and the loss of revenue a beautiful zoo such as Cinci will lose. tragedy all around.