There's video at Channel 4 News UK, showing the gorilla becoming riled up, and dragging the boy quite aggressively, if not violently. No doubt this would have been a terrifying experience to watch, not least of all for the parents. Zoo officials must have been horrified not just at the scene, but at the prospect of killing that prized gorilla, a huge silver-back. See, "A gorilla has been shot dead after a boy fell into its zoo enclosure. The 4-year-old is set to make a full recovery."
More, at NYT, "Harambe, a 17-year-old gorilla, was killed by Cincinnati Zoo staff after a 4-year-old boy entered his enclosure."
And at the Cincinnati Enquirer, "Cincinnati Zoo's 17-year-old gorilla killed after 3-year-old falls into enclosure," and "Video shows gorilla react when child falls into Cincinnati Zoo enclosure."
Plus, "PETA primatologist says zoo enclosure should have been surrounded by a “secondary barrier”", and "Boy who fell into zoo gorilla exhibit is out of hospital and 'going to be OK'."
1/4 Yet again, captivity has taken an animal's life. The enclosure should have been surrounded by a secondary barrier to prevent this.— PETA (@peta) May 29, 2016
2/4 Gorillas have shown that they can be protective of smaller living beings & react the same way any human would to a child in danger.— PETA (@peta) May 29, 2016
3/4 Even under the "best" circumstances, captivity is never acceptable for gorillas or other primates & in cases like this it's even deadly.— PETA (@peta) May 29, 2016
4/4 This tragedy is exactly why PETA urges families to stay away from any facility that displays animals as sideshows for humans to gawk at.— PETA (@peta) May 29, 2016
Still more, at CNN, "Witness: Gorilla pulled the child's pants up; a woman who filmed video of an incident where a zoo gorilla dragged a child throughout an enclosure speaks to CNN."