Was it ever established in the OT that Vader was a genocidal psychopath like Hitler?
Why were the rebels rebelling? Maybe they didn’t like Vader and the Emperor’s tax plans?
Other than the fact that he betrayed and murdered the other Jedi, killed untold numbers of his men onscreen–who knows how many died in the asteroid field-- I guess there is some room for interpretation, but I think it’s pretty clear he’s a really really bad dude.