From what I’ve heard, Lenin’s vanguard party and violent revolution thing was basically theorized to be required basically because of the long history of more peaceful movements being squashed by violent capitalists, the difficulty it is to wrest power from the old dictatorship, that of the rich, and the difficulty it is to change a country’s culture (see the super brainwashed US that might re-elect Trump let alone ever be able to get affordable health care).
Anyone can call any other group “brainwashed.” If that’s all it takes for you to justify violence in order to change a system, you yourself are the fascist. Regardless of how wrong or deluded you think a people are, democracy requires that you rely on debate and conversation to change their minds in order to accumulate the support needed to change the system. If you resort to violence, you are enacting authoritarian rule, plain and simple.