I thought the scene where Chuck cut Jimmy down was beautiful and devastating. "People don't change! You're Slippin Jimmy!" Jimmy has spent years trying to legitimize himself in the eyes of the people who love him and become something respectable, and Chuck puts him in his place, telling him that he will never possibly see him as anything respectable.
Isn't he right though? I mean, he was kind of an ass by not being upfront about it, but I think in general he's right. No matter how much Jimmy seems to want to change, he really does seem to still be taking a lot of shortcuts.
The only reason it seems Jimmy becomes the Saul we know from "Breaking Bad" is because of his brother's betrayal. His brother is a huge asshole and despite what he says, Chuck basically creates the thing he resents Jimmy for being in his mind anyway.
I think the betrayal by Chuck may have pushed him in that direction, but all the traits of Saul were already there, brimming under the surface. Lots of things could've set that off, but ultimately they would only be a catalyst and not a cause. Jimmy always will be Slippin' Jimmy, and for as much of an egotistical dick Chuck is, he was still right about that. And not because it's a self-fulfilling prophecy or because of Chuck, but because time and again Jimmy has shown these negative traits.