For teenagers, these include, not surprisingly, experiencing and/or witnessing violence as children, and the influence of media that depicts violence and revenge as admirable ways to resolve problems.
The media makes heroes out of those who use violence. Local TV-news shows frighten people with a constant diet of stories about victimization. Other factors include our shockingly abundant harvest of guns (20,000 enter the stream of commerce every day), the absence of participating fathers, and on and on.
Violence is the result of a recipe of influences, mixed together in context. Violence is part of the species, and the search for one ingredient that is dominant -- one thing we can blame, one thing we can change -- is a wasted effort. Teenage boys, in particular, are highly sensitive to shame, so challenges to their identity and anything that might reduce their status -- humiliation, threats to their rank among peers, etc. -- are high on the list. Every culture in history has used teenage boys to do its killing.