Luke is a potential threat, the same as any potential Jedi is a threat to him. That's why the Younglings were killed. That's why the Empire began targeting newborn children and those who weren't identified, but have the potential to be Jedi. Even if one or two might be powerful enough to replace Vader. Palpatine was too paranoid about having his Empire fall apart. He said that about Maul being replaced by Dooku and only because he had started the plan and didn't want to wait another twenty years. He was disappointed with Vader, but that comes after twenty years of Vader being his Apprentice. He's not disappointed as we see at the end of ROTS, when he smiles at Vader's reaction to Padme's death. After that, Palpatine never moves to replace Vader when there were enough suitable replacement candidates out there. There is understanding because he let his emotions carry him away after his mother died. It is understandable that someone would take such revenge because of how brutal the crime was against him and his mother. And it in turn sets the stage for how far he will go, when he is not in such a blind rage. Considering that the Tuskens killed twenty six men who invaded their territory and didn't appear to have learned a thing twenty years later, I don't think they care what Anakin did. They themselves were just as guilty as Anakin was.