The learning curve on violence is notoriously steep, and the US left is really not up to the challenge. This is the most heavily armed society on earth, and most weapons are in the hands of the police, the military, and right-wing private citizens, many of whom have very impressive arsenals. Even if the left switched sides on the 2nd amendment, amassed guns of their own over the next couple of decades, and then attempted a violent struggle, the most likely result would be a hellish modern civil war (no clear front lines, terrorist attacks all over the place, a loose coalition of independent left-wing actors which could turn on each other at any moment, and no clear end in sight). They would eventually lose anyway, unless they could manage to take over the state and use its apparatus.
Milder situations of violence, as in the street battles of Weimar, aren't going to go any better for them. It would just result in a series of riots, and the riot police would feel much less compelled to restrain themselves than they are with nonviolent (not to be confused with law-abiding) protesters. Not that they're gentle with ordinary civil disobedience that doesn't involve getting a permit and holding a parade, mind you, but they do usually refrain from using all the fancy military gear that has been sold at a discount to every significant police force in the country. This would go out the window if violent resistance or street battles against right-wing thugs became common.
Violence is of course part and parcel of statecraft, and of human behavior more generally. People who object to it on moral grounds have a much weaker argument than those who object on practical ones. But the practical arguments against attempting a violent struggle are very, very strong.
