The other answers address triggers for violence fairly well. Here's a Web paper by Prof. Suzanne Kemmer that focuses on primal causes of (human) violence.
"But there is no question that violence on conspecifics in humans is often associated with antagonism to particular groups based on perceived 'outgroup' characteristics. The outgroup or "other" is seen as lower on the hierarchy of "us-ness", and this is strongly associated with a propensity to attack. In human societies, the attacks might be physical, but also come in the form of suspension of social rights to protection, food, property, and other rights, so often seen in human history."
tje
2005-12-13 22:02:55 UTC
One cause of violence is blocked social mobility/lack of resources.
In other words, the idea of "pulling yourself up by the bootstraps" just doesn't work and it is the source of much frustration and despair.
anonymous
2005-12-13 13:53:56 UTC
Humans are naturally violent. It allows us to survive. In the ice age needed to be violent to kill animals for our food and if we were attacked by animals we needed to be aggresive to fight back and kill it humans cant use running away as a defense as we are much slower than most animals.
only1breeperry
2005-12-13 17:07:49 UTC
people who just hate each other or can't stand each other.
someone who keeps getting onsomeone's nerves
anthony g
2005-12-13 14:30:59 UTC
people get angry and it has got to go somewere
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