If you are in the developed world, (as was the case here, with the train's route passing through only The Netherlands, Belgium and France) there is no such thing as an 'open carry rights demonstrator', nor indeed of 'open carry rights'; Any civilian in any of those nations who is openly carrying a gun in a public place without lawful excuse is guilty of a serious firearms offence, and subject to arrest and punishment on that ground alone, even if the weapon is unloaded and he has no intention of using it.
If you are in the USA, then your guess is as good as anyone's - if you take the gunman down then you might be a hero, or the instigator of an unprovoked assault.
Civil disobedience has a long standing tradition throughout the entire world especially in the pursuit of new civil rights. How do we know this guy didn't just want to show off his favorite toy in a peaceful way as a part of a civil rights demonstration?
Possession of contraband is a non-violent crime. Is it appropriate for tourists to attack and detain individuals they suspect are committing a non-violent crime while traveling on vacation?
The reports say the weapon was slung over the suspect's shoulder when he was attacked. The suspect was allegedly not aiming the firearm at anyone. The suspect began using his box cutter only after he was attacked by the Americans.
At this point I have only small doubt that the suspect was planning a violent crime, but that doesn't help me when using public transportation in the US and trying to identify threats to my safety.