The thing Obi-Wan never knew was that Palpatine's mind games were more extensive than anyone could have ever guessed.
In order to recruit Anakin Skywalker to the Dark Side, Palpatine took on a new apprentice, Darth Vader, who by all accounts was unremarkable except for his complete lack of scruples. There was nothing Vader would refuse to do, and he was utterly ruthless. The problem Palpatine consistently ran into regarding Skywalker was the young Jedi's constant need for reassurance, which was something he could get more easily from his secret bride Padme than anyone else.
Palpatine thought that if he paired Skywalker and Vader together, it would be a mutually beneficial relationship, at least until the point one would have to eliminate the other, which was how it always worked for Sith apprentices. Naturally Palpatine hoped Skywalker would prevail, given his far greater potential, but if he won, certainly Vader would have proven his worth.
Together, Skywalker and Vader were a formidable combination. What Skywalker refused to do he allowed Vader to accomplish for him despite his continued reservations. Theirs was a relationship of a constant dialogue, Skywalker trying to get Vader to consider his nobler qualities, and Vader attempting to unleash Skywalker's darker impulses.
Over time, Skywalker grew less comfortable with the arrangement, but he couldn't tell anyone. He was ashamed. If he told Obi-Wan, he risked everything. If he told Padme, he would lose her trust. But he'd had a vision of the future, and he feared losing her above all else, and so he continued to trust Palpatine.
Vader's atrocities mounted, and Skywalker became afraid. He became aware that the only way out of the arrangement would be a sacrifice.
Things came to a head on the volcanic world of Mustafar. Padme was giving birth elsewhere, and with Skywalker nowhere to be found it was up to Obi-Wan to watch over the process. She'd been experiencing terrible complications, and the danger to her life was overwhelming. Obi-Wan alone knew who was the father. Skywalker was distracted in every way possible. The guilt of a lifetime poured over him. It gave him great power, but Vader still had the upper hand.
They dueled in blazing heat, lava spewing all around them. This would be a fatal encounter for one of them, and they both knew it.
Up until the moment it happened, Skywalker believed Vader would come around and realize that together they were stronger than either one of them would ever be, and could accomplish wonders, perhaps things neither had ever dared dream before. He had never trusted Palpatine. That was what he realized in those final moments.
The only thing Palpatine stood to gain from any of this was to avoid yet another usurper. Palpatine was a survivor. He knew better than anyone what the best result was.
It was brutal. Vader was without mercy. He carved away at Skywalker until there was practically nothing left, and then watched the rest of it burn. How this had happened to one of the most powerful practitioners of the Force would be unimaginable, except that Palpatine had done everything to undermine him.
For the next twenty years, Obi-Wan convinced himself of many things, once he found out what happened on Mustafar. But the truth wasn't one of them.