On the one hand, I have similar reservations re. James' argument as you do - on the other hand, however, I think that the dichotomy of systemic (social / political) vs. individualistic (going postal) explanations is quite limiting, if not false. Going postal is not an individualistic act, but re-enactment of a cultural script showed ad nauseam in movies, video games, popular fiction etc. Nor is it a purely "systemic" act since cultural tropes with hyper-violent contents are quite widely spread around the world, yet not necessarily produce hyper-violent acts on a massive scale (cf. Japan). Moreover, relative absence of hyper-violent cultural tropes may coexist with quite violent nature of social interaction (cf. Russia).
My explanation of hyper-violent acts is grounded in social control theory of criminal behaviour which in a nutshell argues that delinquency (and by implication hyper violence) is triggered by social alienation i.e. individuals having no stake in community of which they are a part and thus not responding to informal social control mechanisms employed by community to curb delinquent behavior.
The half assimilated immigrants that James describes fit this model quite well - they are alienated from the community of their parents - and thus not subject to strict social controls exercised by that community - yet not fully integrated to the Western community and thus not subject to social control mechanisms that it employs. Hence their propensity toward delinquency and hyper-violence.
An additional element is socialization into a delinquent counter-culture (cf. Jack katz, _Seductions of crime_) that treats delinquency and violence as virtues. Thus, members of such counter-culture engage in acts of wanton violence to prove their own worthiness to their fellow travelers. Alienation and socialization in delinquent counter-culture usually go hand in hand - alienation enables delinquency and violence by weakening 'normal' social control mechanisms, delinquent counter-culture provides positive motivation.
>From that perspective, going postal (or blowing up buses) is both social and
individualistic. It is a product of social conditions characterized by a
high level of alienation and creeping delinquent counter-culture , but it is
also an act of individuals frustrated with the mainstream society and
seeking self-affirmation through delinquent acts against that society.
Politics and ideology - fascism, Islamism, Maoism, racism of one stripe or
another, etc - are mere window dressing and an excuse rather than a
motivating force.
Wojtek