Bernie's not responsible for the conduct of these people, obviously. I don't think the onus is on him to drop out because of a few assholes, though I do wish he'd step up and disavow this.
Actually he is. His campaign has been inciting these crazies with all the talk about how "rigged" the primary is or their whining about superdelegates and closed primaries.
And why are some people surprised that Sanders hasn't denounced these thugs? He just became a Democrat for convenience, he doesn't feel any commitment to the welfare of the party. He wants to keep them as a personal army, so he will continue to tolerate this kind of behavior. Who knows, maybe he will make a statement too about how passionate they are and how much they love their country.
No, he isn't. He never incited violence or told his supporters to behave this way, unlike Trump. And he ran as a Democrat because he vowed not to run as an independent and split Democrat voters in the general.