It seems like there may be a misunderstanding in your question. Obi-Wan Kenobi did not turn to the dark side in the Star Wars saga. In fact, he remains a dedicated Jedi throughout the series, even after the tragic events involving Anakin Skywalker.
After the fall of Anakin to the dark side and the ensuing conflict during the Clone Wars, Obi-Wan faces immense sadness and loss. He grapples with the responsibility of having to confront his former apprentice and friend, who he considered like a brother. The emotional weight of having to kill Anakin (as he does in "Revenge of the Sith") pushes Obi-Wan into a state of mourning and solitude, but he doesn't succumb to fear or darkness himself.
Obi-Wan's resilience comes from his commitment to the Jedi Code and his belief in the light side of the Force. Even after losing Anakin and experiencing the devastation of the Jedi Order, he remains a beacon of hope and goodness, ultimately taking on the role of watching over Luke Skywalker to ensure that the Jedi legacy continues.
If you have further questions about Obi-Wan or specific story details, feel free to ask!