New political currents hate ambiguity and nuance. If you're trying to motivate people to rise up against the Empire, the last thing you want to hear is people saying "Okay, but some imperial officers are actually good people, and this Saw Guerrera is a sadistic monster", even though both are true.
Eventually, once the Rebellion becomes widespread enough, people start saying "Okay, the Empire was awful and we're glad it's gone, but maybe we should cut back on the whole hunt-down-every-single-ex-stormtrooper-and-murder-them-and-their-family thing".
I hope, anyway.