I wonder what round he used (probably never find out, unless Mas Ayoob writes about it), the civilian ones are loaded with Hornady V-Max bullets, better known for their terminal performance on varmints than people. He might have gottten his hands on the LEO or Mil rounds more geared for penetration.
Either way, given the observation by the FReeper you mentioned, this wasn’t “sudden jihad” it was coldly premeditated.
I hope we learn about the bullet type in due course.
Anyway you look at it, this was deliberated and coldly premeditated. Even down to giving away Korans on D-day.