Arthurian Tales type stuff. There's only one problem with that. Arthurian Knights also fell "to the dark side." The bough that refuses to bend, instead breaks.
Yeah, they got nearly annihilated. By the army of cloned child soldiers they decided to use to crush a rebellion against the Republic. After they failed to perform coup d'état against Senator Palpatine. For a legitimate reason, but it's not like they tried to let anyone outside the Order know that.
Yeah, that part's always bugged the hell out of me. But it's not just the Jedi - literally nobody in the Republic seems to have a problem with breeding a race of cannon fodder to fight their war for them.
It's not as though doing so would have made the slightest difference. This is the guy who successfully orchestrated both sides of a galactic conflict. Going up against him in a propaganda campaign would be a hopeless gesture.
They didn't exactly breed them themselves. It was more that the Republic was getting its butt kicked by the Separatists until Obi-Wan discovered that someone had ordered a clone army in their name and no-one looked a gift Trojan bantha in the mouth.
Even if they couldn't win the propaganda war, it probably would have been good for anybody who rebelled against him to know that he was a Sith lord who could shoot lightning from his fingers.
And how do you tell them? Even if they'd broadcast it across the galaxy it would've been just another rumour spread by the remnants of a traitorous organisation. 20 years later, Imperial propaganda has been so effective that practically nobody even knows what a Jedi is.
Which in and of itself is dumb. Even among humans, 20 years isn't enough for most of the race to die out, let alone forget something that was common knowledge. And then there are the myriad of races that live even longer. Almost everyone would still know what a Jedi was, propaganda or no.
