A True Believer Strikes Again
I do not think that this person is crazy - certainly not crazy as in "he didn't know what he was doing". He took years and a considerable amount of money to plan, prepare and execute his attack, so he knew very well what he was doing.
So why he committed an atrocity anyway? Because, I am convinced, he is a true believer.
Not in any formally recognized ideology or religion. No, his own creed was a home-brew from many different sources and authors - which now are often being dragged through the mud. His inspirations are vast and varied, from across pretty much all the political spectrum.
But the heterogeneity and possible contradictions in his creed were not an issue; in the end the mixture was potent enough to convince himself he had a mission to complete, a mission vital for the destiny of the world - his world, at least.
And that's why he went on to kill and destroy. Because he was convinced he had to act, he had to provide a performance that would change everything and motivate others to fight for the cause as well.
And the carnage, in his vision, was necessary. Unavoidable on the way to a new and better world.