Not sure what your personal beliefs are, but in the interest of covering all sides, I will give you my opinion as a Christian.
First, I do not agree with Mr. Sparkle, as good as his laundry detergent may be.

I believe mankind IS meant to be the dominant creature on Earth, as evidenced by what God said:
God blessed them and said to them, "Be fruitful and increase in number; fill the earth and subdue it. Rule over the fish of the sea and the birds of the air and over every living creature that moves on the ground." - Genesis 1:28
Anyway...through Adam and Eve, sin was introduced into the world, and it was at that point that humanity as a whole became cursed. We are all born with an inherent sinful nature. People are born with a nature to tend toward certain types of sin...for example, lust is a sin I struggle often with, while others have very little problems with it. Some people are born with a nature that makes them violent, I am not one of those people (I'm more of a doormat ^^;; ).
Also, and it's interesting that this coincides with the topic, when a human's survival is threatened, morals and decency seem to go out the window. Just look at what people are doing in the wake of Katrina to see that. People looting and murdering and raping, shooting at rescue copters...it's pretty sad.
I'd have to say that violence doesn't bring joy to most people, but that they feel it is necessary in the interest of self-preservation.
So I'd say both those answers you gave are correct. It is due to both the fact that we live in a fallen world (our sinful nature) and also the situations that we get thrown into that make people act in violent ways.
Well, there's my two shiny pennies.