Perhaps Obi-Wan's decision to let him live was meant as a repudiation of the "he's too dangerous to be left alive" sentiment that pushed Anakin to kill Dooku, and Mace Windu voiced when he was about to kill Palpatine.
I never bought the whole transition-to-the-dark-side scene. It was going great right up until Anakin swore allegiance to Palpatine and agreed to hack a bunch of kids to death, and yet still seemed so conflicted & unconvinced that the Jedi as a whole were evil, in spite of what he just went through with Windu (and of course that should have sent up a red flag, since he himself went through the same thing with Dooku.) I know he was supposed to be all besotted with Padme and the thought of saving her through Palpatine's dark teachings & shit...but honestly, I felt it should take a little more than that.
I'd have believed it more if they played up his more fascist personality traits so that Palpatine's course of action actually seemed to him like a good idea, rather than as a means to an end. They really should have made him more of a flat-out asshole, rather than just a whiny bitch (of course, you could probably blame Christensen's acting for that.) I just don't accept that he was wholly pushed into it by the Emperor's promises of power or led into it with Padme holding his dick. Nobody that wishy-washy gets to be Darth Fucking Vader.