Damn, a lot of dark shit must have happened to him to turn him to the dark side. Canonically, Obi-Wan is probably one of the most "jedi" Jedis, even his fighting style Soresu is almost exclusively defensive.
As someone running a soresu based jedi in a ttrpg, there's a lot of times I get real frustrated not having a solid high damage finisher, which does lead to dark thoughts soooo.
Yeah, I mean, maybe in this scenario Qui-Gon didn't die and took Anakin as a second apprentice so that Obi-Wan would be constantly frustrated by being upstaged by that "kid" who is naturally stronger in the force, which will lead to his downfall. His final act of turning would be killing Qui-Gon. Although the Emperor wanted Anakin as his apprentice, he wouldn't be able to turn him and take Obi-Wan as a consolation price, still managing to establish the Empire with Order 66.
And if Maul returns like in TCW, Palpantine would definitely use it as an opportunity to pit his apprentices against each other since they're more closely connected that Anakin and Maul. He'd absolutely leverage Satine's death the same way he leveraged Anakin's fear for Padme.
Obi-Wan is a self-contained dichotomy. He's simultaneously very lax with applying the rules to his apprentice, in a similar way to Qui-Gon Jinn, but also incredibly strict with following the will of the council, even above the Will of the Force.