I think he was totally justified, and in a sense, a march on Rome to preserve his dignitas is completely Roman, Dignitas being a Romans most important quality among his peers. I think he did probably want surpreme power, at least for a period, to push reforms he wanted onto the agenda, but his clemency (ultimately costing him his life) shows that he only followed the example of Sulla in terms of the march, not in terms of a reign of terror after the fact. He was just misguided in his expectation that the oligarchs would "come around".