As obvious as the day is long - he was carrying a "backpack nuke" and was preparing to detonate it.
Two things:
1) let somebody else carry the backpack nuke
and
2) don't expect every attempt at surrender will go without a hitch when the goons come to your door at 0500.
The gratuitous shotgun reminds me of "Harry and the Hendersons"
"Shoot him again"
"I think he's dead"
"Shoot him anyway"...