There are any number of factors that can help lead to violence in a society. We cannot necessarily know for sure which ones are most important.
One cause of violence in society is surely just human nature. Human beings seem to have a capacity for violence that is innate. It seems that society must work hard to prevent these violent tendencies from being expressed. This seems like the greatest cause of violence in society as it helps to explain why there appears to be violence in essentially every society ever known.
There are aspects of society that can help to bring people’s violent tendencies to the fore. For example, inequality in the division of resources can lead to violence. People may react violently when they see that they are not as privileged as others in terms of wealth or status. For this reason, violence can result from a lack of economic and social opportunities.
Violence can also result from things that lower the social barriers to violence. For example, the abuse of alcohol can lead to violence as it causes people to lose their inhibitions. Viewing of violent movies and video games is thought by some to lead to violence for the same reasons.
Overall, then, we can say that violence is innate to human beings. Societies try to erect barriers to violence, but factors such as inequality can make people even more prone to violence while things like alcohol and the media tear down the barriers to violence. Violence results, then, from a combination of these and other factors.