a severely confused and self-loathing person latched onto extremism as a way to vent. i'm not sure that the particular ideology matters here; these people will find radical islam or neo-nazism or anti-whatever and use it as the conduit for their anger.

i'm sure that further gun regulations are a helpful step. at the very least, it would've been much more difficult for him to commit this crime if he were not able to obtain those guns legally. a different weapon may have led to much less bloodshed than a semi-automatic; denying guns might save lives, even if it doesn't save all of them.

the root cause, though, seems to be mental illness. i don't know how you prevent and treat mental illness, but dealing with those issues seems like the most important thing to do.