For example, being part of, and supporting, a genocidal regime is not necessarily the same as committing genocide oneself. (Just as aiding and abetting a violent crime can be distinguished from the violent crime itself.) Sure it’s bad, but not as bad. When it comes to the destruction of Alderaan, Tarkin was the far worse actor.
In the Original Trilogy, Obi-Wan tells us that Vader helped the Empire hunt down and destroy the Jedi Knights. In other words, he didn’t just support the Jedi purge, he actively participated in it and killed an unspecified number of Jedi. So the fact that he participated in a genocide is something that was already established by the Original Trilogy, from the very first movie no less.