It's evolutionary. Aggression breed's through generations. The problem is as a civilized society we are actually inhibiting aggression. Science has proven that stuff like boxing or physical exercise lowers stress. Anyone could go swimming and come to the same conclusion. We have a society where even a parent spanking a child is abuse, which is understandable if the parent is hitting their child out of frustration. But I'll tell you what, if when I have a child he steals, I'll take my chances. If my child hurts another child, I'll discuss what they did wrong and have them account for their actions, and either resolve and apologize with the other child or teach them to just ignore the other child as a stranger (not ignore like I CANT HEAR YOU). Each action should be judged on the same merit, and parenting doesn't seem to go by that. It's either too much or too little and sadly in this world we have few outlets. We're eliminating PE, we're eliminating after school and extra curricular activities. When all a person has to look to is academic success to become a cog in the world instead of having many different options, y ou have a rat. When a rat has no room to vent, he will become stationary and build contempt. You include the internet which has faceless bullies and increases the bullying they get at school when they are different, you run into these issues. Point is, institutional schools are a new thing as well.
After all, the amount of students being taught has gone up enormously, while before it was single room classrooms or classes held at church or no school at all. Now we have ALL kids going to school, which is great, but that also makes it so everyone is at least literate. And depending on the environment they are in outside that classroom, their ideas will start to diverge from the others. Afterall, literacy breeds free thinkers and that could mean they see reason in such disgusting actions.
Sorry, I'm doing a lot of work for finals and I'm sure my whole post is all over the place.
EDIT: Also, I have been playing video games since I was 4 years old. First of which was Mortal Kombat and Duke Nukem. I will state it drives away my attention to my studies, it can even be frustrating. But I'll also say it takes me away from my stresses to watch different stories play out, it's a lot different to be a hero. I am currently playing Star Wars The Old Republic, in it you can be Republic or Sith and be Good or Evil regardless of the side. I have yet to find ANYONE on Sith side that is always dark or even mostly dark. Even on the Sith side most people choose good side options, which bewilders me because to me I want to see what sort of trouble I can get into out of curiosity.
Thought that would be interesting to think about, games that give moral choices and people choosing good even when they are even fictionally supposed to be cold and evil.