Suppose he had been grazing his cattle on land belonging to one of his neighbors, and had refused to pay for the privilege. That would clearly have been theft — and brandishing guns when someone tried to stop the theft would have turned it into armed robbery. The fact that in this case the public owns the land shouldn’t make any difference.
So, what happens if two ranchers want to use that land? Or if some rancher wants to graze his herd on land that Mr. Bundy owns and isn’t using?