One thing appears clear already. He did not just flip. Nor did the killer in Nice "just flip". In both cases it appears some long term planning took place. In Sonboly's case he needed to acquire the gun, and also set up a fake Facebook page. So it is not something that happened over-night.
Now, it does not necessarily have to be connected to his religious or political faith (as with Anders Breivik) but the witness satement from the MacDonalds restaurant where he started his rampage, makes it very likely it is.
I don’t doubt that he planned it, and maybe you’re right.
It’s McDonalds.