So far, so horrifyingly American. But there's an unpleasant local angle: according to his marketing material (example; there are plenty of others), Grossman has trained people in New Zealand:
Col. Grossman is one of the nation’s leading law enforcement trainers. He is the author of the Pulitzer-nominated book, On Killing. He has served as a trainer and keynote speaker for all major national and international law enforcement training organizations and has taught the representatives of literally thousands of federal and regional agencies in the U.S., Canada, New Zealand, and Australia.
[Emphasis added]
Which raises the obvious question: has he trained police here? Has he been training New Zealand police officers to be psychopaths and murderers? I think the police - and other government agencies - owe us some answers here.