What Cassius and Brutus failed to realise, or stupidly ignored, was that the process of senatorial murders, of mob killings on the street sanctioned by the government, of arbitrary death sentences and civil war had changed the world they lived in. They thought that they could be classical heroes like the Brutus who overthrew the kings, forgetting that Brutus overthrew the kings with the help and consent of (almost) every single other senator, having made his case in public in the Forum.
