The shooter is not insane. He's evil. This applies to this guy, Anders Breivik, the Paris shooters, and any number of others like them.
Every one of those people was radicalized.
First, the "other" (to that person) was demonized, then dehumanized.
Once you think of the entire class of the "other" as evil and sub-human, then you've paved a smooth road for a violent response.
Many people are susceptible to being goaded to go down that road. Sometimes, they shoot up Parisian nightclubs. Other times, they shoot up mosques. The human psychology involved is 100% identical. It's not about the underlying politics, or religion. It is how those are USED to radicalize.
Dehumanize, demonize.....then kill.
This is why people have been sounding the alarm when we see and read rhetoric - regularly, and in the mainstream -- that does just that. One of the highest-rated shows on Fox News is hosted by a man who called iraqis "semiliterate primitive monkeys." Dude. That's dehumanization 101. Our sitting president leads off with his reference to Mexicans as "murderers and rapists." The "thought leaders" spewing their filth regularly say the same about anyone from the arab or muslim world, or anyone from latin america. Dehumanization and demonization are tools of arson. Eventually, they succeed in creating a fire that kills people.
There are plenty of ways to talk about immigration that don't have to devolve into dehumanization. But very few people seem interested in having that conversation. And those who have happily traded in dangerous rhetoric are now trying to run away from taking any responsibility for their recklessness. Nope. Own it. Watch it. If you are a muslim radical leader condemning the jews over and over again, you damned well have responsibility when someone watching all of your videos goes out and attacks an israeli bus, or a synagogue. If you are spewing white nationalist demonization of the brown people, and about how their mere presence is an assault on our civilization, you damned well bear responsibility.
Sane, evil people do predictably horrible things when they are spurred on to do them by dangerous and hateful rhetoric. We should stop the hateful rhetoric.