I agree with you that he went over the edge of sanity. Also, his clear thinking eroded when he decided that killing innocents would help any movement against Marxism or Islamism. He lost the ability to see the final cause & effect of his actions.
I think an erosion in his thoughts can be glimpsed here and there even in the earlier writing of the manifesto. The first glitch I noticed was an enthusiasm for Neitzsche. The second was the growing embrace of ultra Nationalism and his comparing himself to Sitting Bull. Not overly alarming in and of themselves. But it is more logical to progress from the philosophy of Neitssche and the massacre at Little Big Horn to killing 90 innocent people than it is get there from a defense of Western culture.