Cabin crew are of course the first line of defense when defusing a drunk...
but when it gets serious, that guy really IS dangerous! other passengers will be inspired to action. Both the chap wearing the hot pants on the way to Detroit, and the jihad inspired idiot who tried to light up his shoe, were brought under control with the assistance of other pax.
I made a point of crossing the pond on UA on 9/11/02 from LHR to Washington Dulles, the plane was half empty. But it felt to me like I was making a statement of sorts, and so did everyone I spoke to on that journey. We were not going to let them win.