I think he initially started simply as a challenge - he appears to have developed an interest in this after several Islamic mass shootings/killings in concert venues last year - and then, either intentionally or through seriously bad luck for us, met somebody who encouraged him to “give some meaning to his life” by converting and becoming a jihadi.
ISIS claimed he converted 6 months ago, which I think would have been on his last cruise to the ME, but that was several months after he started scoping out hotel rooms that overlooked concert venues.
I have no idea. I have no heard a peep from law enforcement regarding the motive.
He bought 33 guns between October 2016 and October 2017.
Also it has been reported that he had over 200 financial transactions reported to FINCEN since 2014.
Those transactions are reported because the can be red flags for money laundering and/or terrorist financing.