I don't think the specifics of his viewpoints matter all that much. Its like the manifesto of a 15 year old school shooter, you only need to see enough to identify that this person is a wacko that doesn't represent any mainstream school of thought or was motivated by one, but rather has their own insane worldview and obsession. Remember that a couple years ago a prominent well respected civil rights attorney who focused on Men's Rights, was murdered- along with the husband of a judge in such a case- by an avowed misogynist crackpot who failed at being a civil rights attorney and filed nuisance lawsuits and his motive was an obsession with the guy he thought stole his thunder. The one was making serious challenges in court to high profile laws, the other wrote million word essays into the void of a zero view webpage where he wrote how he wanted to murder every whore in america. Its an example where the significant movement- mens rights- wasn't really impugned in any way by the actions of the nutter.
This guy was an atheist, an ex-muslim apostate who obsessed to an obviously paranoid and delusional degree over his nutty strain of anti-saudi anti-german anti-everyone-who-isn't-him worldview. He wrote about how he thought the government was spying on him by going through his garbage. He was a whackjob.
The actual failures of the government in this case are a lot more relevant. The pillars put up to protect the christmas village from car ramming attacks had large openings for emergency vehicles to pass through. He simply drove through one of those gaps. What was the point? The german authorities were warned numerous times by the Saudis and even private citizens all who knew this guy was going to commit a terrorist attack, with direct warnings of an imminent attack provided directly to an uncaring bureaucracy that disregarded it all. Even though the guy was blatantly extremist with one glance at his social media, he was given asylum and eventually citizenship when the Saudis wanted him for a laundry list of crimes. The government doesn't have the excuse of being irrelevant or insane
I wonder how much of this has to do with the government being simply unable to keep up with the number of such complains or not having enough to action and actually do something. They've literally let in hundreds of thousands of potential perps. If this guy didn't commit a crime, can they simply arrest him based on some schizo tweets? Lets say those private citizen complaints did make it to some counter-terrorism unit, again what are they arresting and charging him with? Do they keep permanent surveillance on him since they don't have enough to arrest or what?
Not asking this to absolve the gov't obviously they fucked up, but at the same time when Germany has let in millions of Muslims from Syria, Iraq, Afghanistan, from plenty of terrorist hot spots, it becomes a herculean task to prevent shit like this IMO.