In the episode Falling Toward Apotheosis, the piece of Kosh that was inside Sheridan came out of him to fight against Ulkesh. The plan was to get Ulkesh off the Babylon-5 station so the Vorlons would not interfere with Sheridan's plan to remove the First Ones from the galaxy.
Why did Kosh turn against its fellow Vorlons and fight Ulkesh?
The interview with JMS in the lurker's guide for this episode does not answer this question. It poses this along with related questions.
Kosh almost certainly knew of Sheridan's plan to expel the Vorlons, Shadows, and all the other older races from the galaxy. He was inside Sheridan and could sense his thoughts and see what Sheridan saw. That implies Kosh gave his life willingly to not just stop another Vorlon, but to stop the entire Vorlon plan. He was also willing to kill another Vorlon.
The Vorlons and Shadows have been playing their game with the younger races for perhaps millions of years. It's hard to believe that if Vorlons have been doing this for such a long time, one would not disrupt that entire plan on a whim.
Let's imagine how Ulkesh might react to this. Ulkesh has known Kosh for at least a thousand years or perhaps even much longer. I can imagine Vorlons living for hundreds of thousands or even millions of years. The two of them worked together before during a war among the younger races. They've probably been best buddies since a million years ago. Then a piece of Kosh comes out of one of those upstart humans and attacks him! Ulkesh would probably see Kosh as a traitor favoring the primitive races over his own people.
It's hard to imagine the psychology of a long-lived race of seemingly godlike creatures. It's also hard to imagine one turning against its own.
Is there any canon source that describes Kosh's thinking prior to this fight? Something that says it was disgusted with how Vorlons and Shadows play games with other races?
Like my other questions, in accordance with my "Quotes Get Votes" policy, please provide quotes, links, or citations from canon sources in your answers. Sources may include statements by JMS or others associated with writing or producing the show, B-5 episodes, B-5 TV movies, related shows, and novels. I prefer answers supported by original sources over mere speculative answers.