I am not so big on this the extremists are just crazy guys and the ideology and influences doesn't matter at all.
What do we mean by crazy? Are we talking about psychopathy? Because if we are then this guys was not medically insane, neither was he found to have any medical mental illness.
It is a red herring, that anybody who commits mass murder is just called crazy, because normal people don't do it.
Some people are not mentally insane, and they commit acts of mass violence because of their beliefs and ideology.
Most Nazis officers who gassed millions of jews were not insane/crazy people. Most of them has families and right after they killed jews, they went home and played with their children.
By the same token would you say, ISIS is just a bunch of crazy guys and nothing to do with ideology, just because they say they are?
We don't have to dismiss every act of extreme violence by saying just a crazy guy and it has nothing to with his beliefs. Beliefs matter, and they dictate actions. And one doesn't have to be mentally insane to have horrible beliefs and act upon it.