The only creatures I can think of that don't need a REAL reason or justification for hurting, killing, or manipulating each other.
Other animals do what they do for survival, territory, or competitive reproduction. They may act vicious, but none act that way for fury or a simple desire to kill, especially not to their own species.
This, of course, only applies to very few people; such as serial killers, terrorists, war criminals, etc. It's still kind of interesting to think about, especially if you're... just a freak.
Attention all humans, you can see why I spend more time posting online or out in nature than conversing with you now.
Truth is, I'm kind of afraid of all of you.