Well I would guess the chimp felt uhm threatened by these people... I do believe that in some countries humans do attack chimps either for food or some other horrific reasons. If I was a chimp and from one of these countries I would really do more than that if i can to any human that comes close to me I guess.

But also, even dogs can easily kill humans and do what you just mentioned as well, more effectively actually, like the pitbull for one.

So, I guess its not a far off thought to have a chimp as a guard at your home huh? I mean you can make him wear a security costume too with a batton and he can attack intruders.