In the OT, Vader only ever directly kills enemy (and sometimes allied) combatants. Sure, you could argue that he killed Owen and Beru, but he never stepped foot on Tatooine and those executions were probably the purview of his underlings. Similarly, the Death Star was Tarkin’s project, a project which Vader clearly disliked on a deep and fundamental level. Of course at some level, everyone who supported the Empire had Alderaan’s blood on their hands, but to say that Vader had some unique and singular responsibility for Alderaan seems to be a stretch.

All of this is to say that Vader’s portrayal in the OT gives the vibe of a ruthless military officer, but one who generally restricts his villainy to the military combatants of the war, real or suspected. It is easy to see how this would have been his modus operandi for decades in the Empire while that Empire continued to fall ever further into darkness as it began to target civilians and institute a reign of terror with an ultimate weapon. For Vader, a military man, to see the organization he followed fall like this would be part of the tragedy of his character.

To then suggest that it was in fact Anakin who was monstrous long before the corrupting influence of the Empire even existed is to strike at the core of everything Anakin is in the eyes of his son. If Anakin committed genocide before the Clone Wars, if he killed children before the Empire was formed, then there is no good man for Luke to save. Anakin didn’t become a monster when he became Darth Vader; Anakin was himself a monster, and the apparent dichotomy between Anakin and Vader in the OT simply doesn’t exist. Anakin as a man was always Vader, and Vader was always a monster.