>It wasn't just "some nervous passengers". The crew felt that was also, and the air marshall they interviewed agreed.

And just what do you expect those people to "feel" if asked? They have to include in their own deliberations that the scardy-cat is a PAYING passenger. They have to include that they'd look very bad indeed if something did go awry during the flight.
Put in that position there's only 1 answer. But the issue is that they shouldn't be put into that position in the first place.
Security has "cleared" the passenger and he can be assumed to have no items that can endanger passengers or crew. If some passenger is afraid of what another passenger might have in mind isn't the simplest answer all-round to have the scared passenger take some other flight? It's the scared passenger's problem, after all, and not that of the innocent passenger who is making him fearful.

A passenger who is afraid of someone else on a flight ought to quietly tell the gate keepers that he prefers to take another flight. It would be totally different if he had SEEN a gun or a knife, but in theory that cannot happen.

How do you think the flight crew and air marshall would have handled the situation of the scardy-cat said he was afraid of being on the same plane with Some guy in uniform or some famous person or if he was sure the person (being feared) was indeed Adolf Hitler?

To let a scardy-cat cause someone to be removed from any flight is placing the responsibility in the wrong place. And in the stupidest of places too!