Young men are weird. They do weird things. I know because I was one and more because I grew up around them. In these terms, I was a particularly placid kid and in truth I wasn't close to many who really fit into this category. But still I know the type. I don't think you can be a boy and go to a junior high or high school and not know it.
When we saw those pictures on Thursday it wasn't clear there would be a foreign connection. To me, frankly, they looked like frat guys. It even occurred to me whether the perpetrators had consciously put on this sort of get up to disguise themselves. Knowing what we know now, that seems unlikely. But when I did see those pictures and see what looked more like frat kids than jihadis or white supremacists the thought that came to mind to me was Columbine -- no clear ideology just the hard underlying precipitate of young male alienation, cockiness and aggression.
It now seems clear that Islamic radicalism played some role in these attacks, quite possibly a lot (as evidenced by the older brother's profile, recent trip to Caucasus and Russia's apparent tip to the FBI), but we need to wait to get a fuller picture.
However that may be, and speaking in general as opposed to about this particular case, I don't think we should see these as mutually exclusive explanations. Particularly in America and with young men like this I think these ideologies are something more like sheaths into which the same young unattached male toxicity is poured.