Well honestly even though its somewhat far fetched, and a long shot....
but... this may be the real motive behind the killings in Vegas... and its really the only scenario I can come up with that still fits.
Australia knows its gun ban is failing badly. So they want guns banned in other areas so gun running stops (yeah impossible reality, but they're desperate since their ideology is failing in the UK and across the EU),
So what would happen if... they set up a mass shooting or used this individual in some way, perhaps gambling debts, perhaps just ideologically. Either way, they want guns banned in the USA too.
What if his girlfriend was actually linked to the Australian government or some other interest there ideologically aligned.
They set up the shooting using this agent, provide all the explosive extras and how to use all the equipment. He doesn't care, he's 64 at this point, not much life left to live.
They plan the shooting, targeting people who would never support gun control in order to flip the board, send the girlfriend off to the Philippines, and wire cash there, since the Philippines are a hotbed of ISIS activity, to misdirect the investigation.
Basically pull it off, and then seed their allies in the news media and Democrat party with gun control stories and talking points, (Such as Silencers and Bump Stocks)... in hopes to coerce the USA to take measures to ban guns.
This would mean our friends weren't exactly our "friends".
Yeah its a theory that's way out there, but honestly its the only motive I can come up with that actually includes all the known players.