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Non-violent means you don't bring a weapon. It is a self evident concept. This does not mean they don't have a right to be armed. I just think - personal opinion - that having arms in a protest turns the vast majority of bystanders watching on TV against you rather than for you.
I think you make a good point. There is an important consideration for how their actions would be perceived by the broader public. In order to win the war to push the government within its constitutional boundaries, a lot more people have to be won over to the recognition about how out of control it has become.
Perhaps we can be more forgiving for their error in judgment. These particular men and women are very idealistic and take their history to heart: the example of the Revolutionary generation inspires them to think that it will take a relatively small percentage of Americans willing to take bold action to bring about change.