I’ve always had some serious, massive thoughts and feels about this scene where Obi-Wan watches the hologram of Anakin slaughtering the Jedi, kneeling at Palpatine’s feet, and everything else that followed.
To begin with, I don’t think Obi-Wan had any idea of what he was about to see. Yoda said he would only find pain if he watched, but Obi-Wan said he “had to know.” To me, he meant, “I have to know how my son/brother/BFF/lover/whatever died.” Because such was Obi-Wan’s faith in Anakin Skywalker that it never occurred to him that Anakin would commit such atrocities. In his mind, Anakin’s body is lying somewhere in the Temple, surrounded by over a dozen dead clones and the younglings he was seeking to protect. He needed to see Anakin’s fate so that it would be real for him, otherwise he’d always cling to some delusional hope that Anakin was somehow still alive.
Then he sees the truth, watches Anakin slaughter their brethren, but what next catches my attention is the moment he watches Anakin kneel before Palpatine. My headcanon is that Anakin has always had some serious issues about kneeling before anyone, going back to his life on Tatooine. Watto may not have been the type to have Anakin or Shmi kneeling in subservience, but Anakin very likely witnessed other slaves being forced to do so before their owners, and it was something that sickened him. That feeling was likely something he carried over into his life as a Jedi, and he may have actually been able to communicate it to Obi-Wan on some level. Hence why Obi-Wan looks like he’s trying not to vomit as he stares at Anakin on his knees before Palpatine. He knows that Anakin despises that sort of posture and behavior – so what has happened to make Anakin go back on that? How did things change so much since he left for Utapau?
This is quite possibly the origin of Obi-Wan blaming himself for all that happened. Obi-Wan clung so tightly to his faith that everything was hunky-dorey that he didn’t see that something was clearly wrong with Anakin, that he needed help. He knew that there was the problem of asking Anakin to spy on Palpatine (see the novelization where he basically tells the Council off for being morons on the subject), but he had no idea of the other issues that were crushing Anakin under their collective heels (the nightmares of Padmé’s death, etc), in part because Anakin didn’t confide in him but also in part because Obi-Wan did not ask. Fanon likes to think that Obi-Wan figured out a long time ago about Anakin and Padmé (because subtle they are not), but he never came out and said anything about it. Perhaps he came to believe that if he had actually sat Anakin down and talked to him, gotten him to share his burdens, then he would have been able to help Anakin before he drowned. We know Anakin did go to Yoda for help about Padmé but got the singularly unhelpful advice of “someone dies – have a party, you should, for one with the Force they are!”, but one would think that Obi-Wan’s advice might have been a little more practical, say something like “Have Padmé get checked out by a few healers, ones who specialize in pregnancy and childbirth, to head off any potential problems.” And that’s just one example.
To continue: Obi-Wan watches his brother/BFF/lover/whatever systematically slaughter their people, and then is told by Yoda that he must destroy him to save the Republic (never mind the fact that the Republic was dead long before all of this went down, but that’s a rant for another time). Yoda maintains that Anakin is already dead, and all that remains is Darth Vader who is basically possessing Anakin’s corpse. In all honesty, I think Obi-Wan kind of went on autopilot at this point, which is why he sounded so wooden and vacant when he kept telling Padmé what a threat Anakin now was. He couldn’t bring himself to believe it, not then.
In all honesty, I don’t think Obi-Wan really believed that this was all happening until he saw Padmé be Force-choked on Mustafar. Because the real Anakin would have sooner gnawed off his remaining flesh arm than do anything to harm Padmé, and Obi-Wan knew that. That’s why he looked so utterly horrified when he stared down at Padmé’s crumpled form (besides the fact that, you know, she had just been choked and that could very well kill her and her baby). That’s when it finally hit home that this was Darth Fucking Vader and not Anakin Skywalker. And I’m pretty sure what was left of Obi-Wan’s heart just shattered.