There are many things that can result in a kid developing a very angry character.
It could be simply a predisposition to be like that by way of genes, or the most common, home and societal influences.
A child brought up in an environment where he is constantly ostracized picked on and discriminated against tend to develop a mean streak.
You can quite easily see how this can happen.
You don't get exposed constantly to certain elements in life without it causing some kind of change to your psyche.
As an adult, if you suddenly are exposed to some adverse conditions and interactions involving people around you, you will tend to bend and adapt to this.
In a child that change is a lot easier to instill and is a lot more permanent also.
So a kid, if exposed to these negative and deleterious vibes, is most likely to grow up into a very angry and destructive individual.
You have heard of kids all the time cracking, acquiring weapons and going on shooting sprees, killing in lots of instances several of their peers.
If you should go back in the past of these unfortunate kids and follow their lives from they were tot, it is very likely you will find some very disturbing influences in their lives that would bend and change even the most resolved of adults.
Then, to add fuel to the fire, if you have an individual who is already pre-disposed to anger and put them in such a situation, you have a time bomb that will go off.
Its just a matter of time.