I think if you step back and look at other animals, what you often see is males competing for females. Most especially in mammals the male is often a bit larger and more robust and must fight another male for his pick of a female. What decides who wins the right to mate is who is stronger, more courageous, daring, etc.. etc..
I think when you take a few millions years of letting the most bad ass mammal have the right to mate, you end up with a lot off spring that have a tendency towards aggression. Every great civilization has had it's great wars. Civilization is not very new, but fighting over resources is not new. Take a look when you throw a peice of bread towards a bunch of birds. Usually there is one ****** who pushes everyone out of the way to get the biggest chunk of bread.
So now we have two instances where being aggresive means survival. Survival in the sense of self preservation and survival in the sense of preserving one's lineage.
Things can change and it isn't always the case that those tendencies lead to self preservation and procreation, but... Even to this very day, those are tendencies which will cause you to excel.
In modern society however, we find extreme cases. People with extreme tendencies towards aggression and self preservation tend to be criminals of some sort, abusive, etc.. etc..
Anyway, without digressing too much. I think what you'll find is that the vast majority of us have these aggressive tendencies hard wired into us. Through the advent of culture which is a product of a particular civilization we learn (or fail to learn) ways to direct this energy in a manner that is productive to the collective whole we belong to. If I decide to go on a murderous rampage it doesn't really help anyone. My lineage will suffer, my reputation becomes tarnished, those who knew me are viewed negatively and thus their chances of finding suitable mates are decreased as well, etc.. etc.. But the one who directs that energy toward some productive end (in terms of strengthening cultural/social/economic ties) increases it's chances of securing resources/food/shelter and desirable healthy fertile prospecting mates.
I think it's quite like the scenario where there is a dream machine. In this machine you become completely connected and enter an artificial dream. You can become any person you like at any time and live out an entire life in the span of a single second then wake up and come back and be yourself again. After choosing all the best lives you can imagine living, would you not be tired of it? Would you not start to become increasingly selective in your choice of life to live. You might decide, I want a calm humble life. You may decide, I want a life riddled with strife and struggle. You may want to live the life of the hated enemy. Perhaps you want to live the life of the victim of some extreme suffering and horrible fate.
I think it's the nature of this universe in general to want to express all the possibilities possible in an infinity of time. And as creatures having sprung forth from this universe, I think we all desire to live out different fantasies and dreams.
I play violent video games, horribly violent. heh, but ya know what gets me? I think it's absolutely despicable that some people enjoy murder mysteries. I find it rather disgusting and appalling that some one could take pleasure in the drama of the after math of a brutal murder.
I think what people don't realize is that the vast majority of us, if not all of us, are really no different than the worst and best person we can think of. 99% of all that separates ourselves from them is circumstance and experience.
Look at the witch trials. ENTIRE TOWNS OF PEOPLE collectively deciding to murder some one who probably just slipped up and went through with an act that everyone else would have wanted to do if they could, but were too afraid of deviating from the cultural norms.
Anyway, I've digressed horribly.
Point is, is that the natural selection of the human species and mammals in general has lent itself to breeding rights for the toughest male. Therefore, we all have a very strong tendency towards aggression in some form or manner. How we end up directing that energy often decides where in society we will end up, and in life in general.