When faced with a being that they don't understand, why is the first reaction of some people to kill it?

We were watching one of those silly reality Big foot shows, where a guy supposedly actually shot and killed a Bigfoot. Of course, there were 10,000 reasons why he didn’t have the body as proof. However, they had actors portraying the Big Foot tribe that was involved, and it was just horrific to me that his first thought, and action was “SHOOT IT!” In the story, the animal wasn’t even threatening him.
Granted, this was fiction, obviously, but some people really are that way and I think that it’s scary, sad and horrible that for some, when they see something they don’t understand, their first thought is to kill it.

Why are they this way? Is it some base instinct that they don’t even think to question?