Instinct, it's what they do.
They are actually extremely brutal and did you know they are one of, if not, the only animal they would rather shoot to kill if it got out of its enclosure?
Lions, tigers, as frightening as they are, a couple of darts knocks them out.
But this guy?
No. The tranquilizer darts don't work fast enough and by the time it did, it could have absolutely destroyed a person or even kill them.
They are vicious creatures, they bite the genitals off, the fingers, maul at the face, bite at the throat, these sons of bitches are going to inflict the most painful things imaginable to someone they attack.
Because it's what they do.
Even Gorillas as big and strong as they are, are less of a threat to you than a chimp.