I agree with most of what you're saying, Tiptup. I really don't mind the children slaughtering, but it needed to have a more compelling reasoning behind it. In the original, if Anakin had convinced himself that the Jedi were evil, then maybe killing children would seem justified. But for Anakin killing children in the hope of saving his wife... I don't buy it.
Again, I don't mind seeing him get very, very evil to the point of not being likeable, because, after all, he IS the villain, no matter what George says now. The problem with the prequels is that they try to make him too sympathetic (even though it doesn't work) by diluting the reasoning for his turn. "His heart was in the right place. He sacrificed himself for love. He was the victim. Blah, blah, blah, blah." No! He's supposed to be "twisted and evil." He was supposed to be "seduced by the dark side of the force." There was supposed to be an impatience and a lust for power. Like he says to Ben, "When I left you, I was but the learner. Now I am the master." He did it for the kind of power he didn't feel he could get from Obi-Wan. I don't mind Padme being the catalyst, because it would work that way. But the way it is now, Padme is all the motivation there is, so, like I said, there's no reason for him not to open his eyes and realize he f'ed up once she's gone. There must be some choice beyond Padme about choosing the dark side. Like Tiptup said, it's all about choice. He could tell himself he's doing it for Padme (after having been seduced by the power of the dark side and resisting it a lot), maybe vowing to use the power once to save her. He does, but then he realizes how wonderful this new power is and refuses to give it up. There we have the wrong thing done for an arguably right reason (in fact, an ethical choice outlined in the definition of a tragedy: a choice between something that's ethically right and something not inherently ethically wrong) but then the choice to keep it when there's nothing at stake except for his own greed. His change to evil forces Padme to leave him, thereby losing the wife he just saved. Now that's much more compelling and leaves him with a much more lasting reason to stay with Palpatine.
I really hate the plot point that Vader spends the next 20 years trying to overthrow Palpatine exclusively. That makes no sense to me. I mean, in the original movies, he seems to be pretty comfortable where he is. Maybe he entertained the notion of becoming the leader himself, but it seems like he was mostly loyal or at least realized enough that he had no chance of doing so. And I always thought the line, "You can destroy the Emperor" to Luke was mostly a trick. But, Jumpman, you seem to think that everything evil that Vader does is just him realizing that it's wrong but he has to do it for his plot to destroy Palpatine? That makes no sense. Obi-Wan and Yoda had a plot to destroy Palpatine too, but it didn't involve them killing innocent people. If Vader was truly "good" the whole time and didn't believe in Palpatine's Empire, but just biding him time undercover, then why would he go to so much damned trouble to destroy the Rebellion, the only opposition the Empire has?