Oh, boy, so many points made... Before I address the OP, a few things about the prequels... I do not believe that the takeaway was that Anakin should never have disobeyed the Order or fallen in love or married. His downfall was that the Order indoctrinated him into thinking that his genuine feelings are wrong and mustn't be there. This is what made him conflicted inside and what led him to submit to the one person who seemed to understand and support him at the time - the chancellor. I'm tempted to get political here since this mirrors a lot of what is going on on a bit larger scales in recent years very well but that is perhaps a discussion for another time... Now, while I appreciate that Lucas is a poor writer at best and that for all we know his message might have been that the Order is sacred and that Anakin betrayed it because of datass, my takeaway was that the Order and the Republic were corrupt and broken beyond repair, profoundly unprepared for as few as two people talking outside of their immediate surveillance. Especially the Jedi are portrayed as not merely flawed but outright rotten because of the position they have been put in for so long and frankly you can see the remnants of that in the Original Trilogy's portrayal of Ben and especially Yoda, too.
Speaking of the OT... And this is coming from someone who in recent years has grown to be somewhat of an OT purist... Again, probably unintended by Lucas himself, the original tale was ripe with layers upon layers of symbolism and while there is a simple story on the surface to tie it all together, the sheer depth of the messages and nuances can hardly be overstated. Darth Vader was what remained of a man after the Empire had feasted on him. He was stripped of most of his humanity by the time we see him, rendered, to put it in Ben's words, "more machine now than man", and of course that, too, is not only meant literally in that Darth Vader essentially exists because of cybernetics replacing essential body functions and big parts of the body itself, but also symbolically in that he is not so much a person with dignity and honour, but a tool, an instrument of the Empire's might. His relationship to the Death Star shows vividly how the two are competing for essentially the same position of the Emperor's public face and his ultimate superweapon, one with the magic of life, the other with technology - a conflict mirrored within the body of Darth Vader himself, too.
So is Lord Vader morally accountable for his actions? Well, this depends. Clearly there was a spark of humanity left in him through to the end, so he was never fully a tool to be used at his master's discretion. But how much of a machine does one have to be before one is clear of responsibility? How much human is enough to qualify as fully human? Are there levels of humanity, can one be more or less human than another? Can this change over time? To the extent that Star Wars presents us with these questions (and I'm happy to say that not all of them were put there intentionally), it doesn't exactly direct us to answers the way the prequels did. There is a reason we bring up Campbell so often when we speak of Star Wars, because at the end of the day, those questions of what is the measure of a man, what are our duties towards each other and towards ourselves, are a typical theme in myths of this sort. This is what they are designed not so much to answer as to address and to make us think about.
Better to leave questions unanswered than answers unquestioned
