I've just read his manifesto, I wouldn't call him insane or stupid. He's an ordinary human being, completely wild and ruthless by the nature and has logical errors caused by emotions. This is the portrait of a true "what would be" situation if humanity decides to ignore law and took action on their own. That's why the entity of state is crucial.
The only thing that bothered me in his manifest, as I understood, he's a person of honor and high pride, he claims he has no problems with Muslims or people of other colors, but he sees them as enemies when they come to his lands and he's driven by the motions of revenge.
I would love to question him what he's thinking about America, which is not Europe, nor was European land. It was stolen from the people of other colors. If he's okay with the idea of United States, he might as well should be okay for Turks conquering Istanbul, which is practically the same occupation in different time zones.
I really couldn't understand his personal disgust to the Turks. Istanbul is a turkish land for many centuries and turks are the least migrating race to Europe. Strange.