I think he brings up some good points, though his murderous rampage served no good purpose.
I was just thinking something similar. This is a thinking man's suicide bomber tactic. He planned an elaborate crime which has focused worldwide attention. He let himself be captured alive, where he will be put on trial, which will also, no doubt, be a circus. His manifesto is out there. His trial will raise attention to his cause.
It's a monstrous, savage way to go about it, but if his purpose is to gain attention for his cause, he's already succeeded. As far as that goes, I'm sure he's happy to be dismissed as a nut and a monster. If can get the world to repeat what you just said, he'd consider himself successful beyond all measure.