Saying "it's just how people are" isn't really good enough, because most of the time, and with most people it just isn't like that. You'd have to lean pretty hard on most people to make them do anything like that. There are reasons why people do seemingly motiveness things like that to animals, and to other human beings.

And I don't think saying "it's because they are evil" helps either. True enough to describe the people who do stuff like that as evil, on the basis of what they have done, but that's a label that arises from their actions, it doesn't do much to explain where the actions come from.