I'm taking the kindest possible view of this situation, which is not easy given my gnawing fear of sectarian political violence in my country. Even is this guy wasn't actually calling for violence, he was being what we on the civil libertarian side of things call a rights troll - someone who exercises a real, legitimate* right in a stupid, shitty way. Yes, you apparently have a right to open-carry in New Hampshire. You apparently even have the right to do so at a public gathering where it could be considered behavior likely to cause a disturbance and, oh indeed, where the president is appearing. You are allowed to do so while carrying a sign reading "It's time to water the Tree of Liberty," a reference to a Thomas Jefferson quote that goes roughly "the tree of liberty must be watered from time to time by the blood of patriots and the blood of tyrants" - a quote which last showed up in our public discourse when a young man named Tim McVeigh was arrested wearing a shirt with that phrase printed on it.
So, yes, as I said, I'm taking this in the best possible light, but it's still creepy and shitty. I get the feeling that, at best, this guy was hoping to be hassled by police/Secret Service so that he could talk about how his rights had been violated and Obama's a fascist and what not. Which is what makes him a troll.
*Actually, I've been reading about Second Amendment history this summer, and the idea that the 2nd guarantees you a personal(and basically unrestricted) right to carry weapons is a fairly recent one (codified into case law by the [questionable] Heller case). You could still make a case for a personal right to gun ownership under the 9th Amendment. More on this later.