The mass killings at the end are like the finale of a fireworks display on the Fourth of July; an orgasmic, sky-filling blast that can only be followed by exhaustion. There's nothing left; nobody's still standing. Even the buildings themselves are dynamited, to make room for bigger and better ones run by corporations.
The movie sounds a final, cynical note about "progress" -- a note Scorsese himself might do well to heed. Casino is GoodFellas II; there's more of it, but it's not better. The three-hour length of the movie calls unpleasant attention to the human smallness of every character in it, and no amount of brilliant acting and directing can make those mean figures seem any greater.