How about it was just plain lazy filmaking. It was in the movie because it was in the script. Anakin has to turn to the dark side.
There is no choice in the matter he either goes bad or the original trilogy events are forever altered or never even happened.
One minute he's a good guy the next he's killing younglings.
The movie and the prophecy pretty much state it was his fate to turn to the dark side, and as it was fated by the force anakin had no free will in the matter because he had more midi chlorians than everyone and was the chosen one, Plus he had daddy issues since he was conceived by the force and had no father.
The Force is neutral it is neither good or evil. Being a nod to eastern Belief. The force did not care about the murders of the younglings. The suffering of billions in the Civil War, it did not care Padme was killed, or that Luke and Leia were put through absolute hell because of their dad. It all simply happened because the force wanted balance because there were too many jedi in the galaxy and only 2 sith.
So basically Revenge of the Sith's message is that people have no free will at all and are only pawns of the fates like humans were in the greek myths. Like Oedipus who by trying to deny his fate actually brings it about. I reject this view whole heartedly because i believe Human beings have free will.
Still i don't think Lucas was trying to sell some overt message it was just in the script. Things had to all go to hell to lead up to events in episode IV, or just star wars if you prefer.
I thought peoples Grievances (no pun intended) with sith were that the turn scenes were basically unbelievable and happened on the spur of the moment. Not the idea of Anakins Fall but the way Lucas botched it.
The slow kind of Build up to a fall for desire of power, the corruption, the addictive nature and Seduction that realistically would have taken several films to lead up to and explain. It was beautifully hinted at and described in Empire Strikes back by Yoda and Obi wan in Return of the Jedi. Why did Lucas have to ruin that. He also ruined the scene of Leia remembering Padme from when she was a little girl on Alderaan. Padme was not originally susposed to die in Childbirth or of a broken heart.
Then again everything about the prequels stink. Vader and Fett and the Mandalores were supposed to hunt down and kill all the jedi. Lucas ruined the mandalorian backstory. He also removed the cloned jedi subplot.
He ruined the fact that Yoda was supposed to Be Obi Wan's Master, then we get to episode 1 and some non existant character named Qui Gon appears.
Jar Jar obviously was never one of the concepts for the original prequels ideas between Lucas and Kurtz.
Anakin was not going to be shown starting out as a little boy in the story or building C-3P0.
The threepio thing must be lucas humor because Vader becomes more machine than man and is dehumanized while the droid he created acts more Human than Vader.
Anakin as a slave thing also is a wondefully neat idea because anakin is never really free. First he is a slave on tatooine. Then a slave to the jedi order and their ethics, then a slave to evil and the dark side and he even gets imprisoned in an iron suit and buries his Humanity within himself. Even changes his name.
The only time he is free is when he saves Luke on the Death Star II and then in death. Up until then he was always a slave in one way or another, like he was a slave to his passions for Padme.
Love and Marriage should be a very positive and liberating thing instead it was another trap for poor anakin. Because the Jedi are not allowed to love. are not allowed to feal. are susposed to remain distanced from emotions.
The jedi rejected Anakin out of fear, and out of jealousy for his abilities. But they also made him reject his essential Humanity. Why should he be forbidden to Have Sexual relations with Padme, Why are the jedi against Nature and the order of the cosmos. I mean its not like he was not Married or was just shacking up with some random whore. Where do jedi children come from if they are not allowed to pass on their dna to children.
I think its great that the old Jedi order and its ways died and Luke started a new jedi order built on love and tolerance. I could care less if it is not canon since it is to me since Lucas is not going to make the sequels. Luke Marries Mara Jade and they don't have to hide their relationship. They are allowed to love and they raise their son Ben.
I mean the title Return Of the Jedi hints that luke must establish a new jedi order with Him as the First new Jedi. Automatically being elevated to Grand Master just like Yoda on the Old jedi Council. And the first person he would teach of Course would be his sister leia.