Harambe, a 17-year-old Western lowland silverback gorilla, was shot after the four-year-old crawled through a barrier and tumbled into the enclosure on Saturday.
Video footage shows the creature dragging the boy through a water-filled moat while onlookers scream.
At one point, the huge animal stops and appears to hold the child's hand before pulling him out of view.
While generally acknowledging that zookeepers had few options, critics slammed the zoo and the child's parents.
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"This tragedy is exactly why PETA urges families to stay away from any facility that displays animals as sideshows for humans to gawk at."
Zoo officials said they were heartbroken over the loss of Harambe, but had to make a rapid choice on how to rescue the boy.
"A child's life was in danger and a quick decision had to be made," Zoo Director Thane Maynard said in a statement.
"Tranquilizing the 450-pound gorilla was not an option. Tranquilizers do not take effect for several minutes and the child was in imminent danger."
"We know that this was a very difficult decision for them, and that they are grieving the loss of their gorilla," the statement said.
Still, angry animal lovers expressed outrage that the child had slipped into the enclosure.
By today afternoon, at least 13 petitions had been created on the change.Org website demanding justice for the slain animal.
The boy's mother "did not give proper supervision. As a result her child fell into a gorilla enclosure at the Cincinnati Zoo resulting in the innocent living creature being euthanized," one petitioner alleges.