I wouldn't call him responsible for the destruction of Alderaan. Grand Moff Tarkin outranked Vader (who didn't actually have a rank within the Empire, he was just Palpatine's lackey.) If Vader tried to stop him then Tarkin surely would have went to the Emperor about it, and it eventually would have happened anyway. But Vader did commit a genocide against the Jedi in the prequels and between Episodes III and IV. I mean, he slaughtered younglings. I know you said that you're only focusing on the original three, but the prequels are all about Anakin/Vader and they are a part of George Lucas' vision.
![[Image: nL4L1haz_Qo04rZMFtdpyd1OZgZf9NSnR9-7hAWT...dc2a24480e]](https://external-preview.redd.it/nL4L1haz_Qo04rZMFtdpyd1OZgZf9NSnR9-7hAWTNVY.jpg?width=216&crop=smart&auto=webp&s=7b11e8b38bea0eacc8797fc971574ddc2a24480e)