First off, "What have I done?" was a reshoot. Second, even if the Dark Side isn't taking over him in that office, that doesn't matter. Palpatine holds the key, or so he thinks. And even with that knowledge, he knows that Palpatine is also a bastard. But, Anakin can't take that chance. He blames himself for his Mother's death eventhough he had nothing to do with it. What person blames himself of his Mother's death when he had no control over the situation? Either way, he does and he makes a promise to not let anything of this nature happen again. Palpatine's offer is the only thing he can grab onto and he can't risk not taking it, even if it's true or not. He has to go forward. He's already committed a sin against his Jedi brothers by help taking out one of the Jedi Masters. He has no choice but to go forward for himself and for Padme. And that's why I think everything after that point is extremely sad.
He would go THIS FAR to save the woman he loves. And by giving into that FEAR, the Dark Side begins it hold over him. Remember, "fear is the path to the Dark Side." And that's the reason Anakin makes absolute no sense, in terms of dialogue, in the rest of the film. Lucas just changed the context when he changed the turn.
Admittedly, it didn't make sense to me either until I saw the film a second time and understood what was really going on. And despite the fact that from the turn toward the end of the film was concieved differently, Lucas made it work, obviously from where I stand.
Lucas just felt that Anakin turning out of feeling disrespected and betrayed fell into that trap of thoughts many assumed would happen when he eventually got around the Prequels.