Good writing is going to present characters with opposite ideas as well as a given character with conflicting morals etc. It allows the author to argue both sides of the equation. There is no right answer. Here is the argument that from the beginning Jimmy is a simple soul, was honest and loyal and not inherently Saul Goodman unlike Walter White was inherently a bad guy.
Jimmy ran small time scams, largely dependent on the greed and dishonesty of the mark and later on marks that were a-holes kicking around the service people or taking advantage of simple people. This largely amounts to cutting corners, taking the easy way, being a rabble-rouser cause the world is not as it should be. It is NOT con jobs such as those depicted in ‘House of Games’ or ‘Confidence’, for example. Not purely malicious manipulation either, a skill he posses that can cut both ways. It is key that Jimmy left this world and his friend to work in a mail room with an often-expressed desire to leave slipping Jimmy behind. It is key his friend stated Chuck doesn't even like Jimmy. That tells you Jimmy is not inherently a con artist like Walter White was inherently a bad guy. Where White was prideful, revenge driven, petty, and mean (e.g. his notes to students that 'this is ridiculous, apply yourself' etc.), Jimmy wanted his brothers support, aspired to move beyond his lowest domination, did things that jeopardized himself for others. Even when Jimmy goes back to Cicero, and realizes this is not who he is anymore and is going back to the mail room. Even so, and clue #2, he runs one last scam for his friend because it means a lot to his friend not because he wants to do it. He is loyal as the ring from his friend’s mother indicates, it always reminds him of a friend, simple like him, who had the best week of his life with Jimmy when he came back.
There are countless examples of the nature of Jimmy. He set up the event with the other lawyer to fix the senior problem and knew the seniors would hear it so the one senior would not be ostracized anymore. He did not have to do this, no one knew, yet he did at the cost of getting the money he desperately needed from the settlement and of the cost to his reputation. The seniors now shaming him is a high cost for a guy who values being liked and top it off gives HHM the win. Triple loss to do the right thing. With Mesa Verde he did that for Kim not himself. His brother went out of his way to snag Mesa Verde because he was jealous of Jimmy. Jealous the mother liked Jimmy more, his wife thought Jimmy was funny, Jealous Jimmy had a girl and he is alone. Chuck then set him up in the most hurtful and deceitful way entrapping his brother and materially affecting his ability to support himself. All for petty jealousy.
What more do you need to counter this narrative that Jimmy, in the last scene of the episode of season 4, where he dups the board and this means his is finally shown as the con who uses and manipulates people, that he really is at heart, Saul Goodman? That is so completely against what is shown in the series. Aside from the examples already sighted (goes to work in a mail and then goes back to work in a mail room; does one last scam for his friend; the senior thing…) he is also shown taking care of his brother, the same brother who gave him zero authentic kudos for passing the bar, told Howard to pass on the senior case rather than hire Jimmy, a case to help seniors getting ripped off, a case he dug around in garbage cans to make. The same bother who told him shortly before he killed himself that Jimmy never really mattered all that much to him, to just accept himself as the looser he is, with Jimmy sincerely saying he can change. All out of Chucks jealousy, that the career, the reputation, the esteem in the law industry, all of his accomplishments and none of it could make people like him, want to be with him, love him like they like and love Jimmy, like their Mother who called out for Jimmy on her death bed. The flash back scene where Chuck is the sponsor at Jimmy’s swearing in ceremony and then the karaoke bar is not a flash back, it is a fantasy. It is a look at the simple thing Jimmy wanted, acknowledgement from his brother. No way Chuck, who is already shown by his Mother’s death bed with the Mother asking for Jimmy or his then wife laughing at Jimmy’s lawyer jokes but not Chucks, no way this guy is attending his swearing in ceremony let alone a karaoke bar and then putting a puke can for Jimmy, offering breakfast the next day and singing themselves to sleep. The only true thing in this scene is Chuck taking over the singing and Jimmy stepping back glad that he accomplished his goal, his brother is having some fun. This shows you what Jimmy wanted and didn’t get and would never get from Chuck or the system and its politics. Truth, honesty and loyalty do not matter to either one. Despite being honest at the first hearing to get reinstated, didn’t spin some BS about Chuck to please the board, it didn’t even occur to him consciously (and ironically, they call this insincere…) and deny him. Despite the campaign Jimmy and Kim put on and despite the planned effect of the letter, none of that would have gotten him reinstated and instinctively he knew that was the case. So out comes this guy, the guy Jimmy could be but did not want to be, the guy who is righteous, caring, loyal uses those same gifts, in some Darth Vader move to the dark side, and goes the other way. That is the guy they asked for, that is the guy they believed and that is the guy they reinstated. And the letter? Nonsense. Chuck would ever write that letter. More likely it was a letter with one more knife to the heart from beyond the grave.
Kim’s shock is not witnessing some facade stripped away. The shock is that her connection to the good, the righteous, to meaningful just disappeared, right before her eyes and in its place the complete and total transformation to Saul Goodman. Like he put on a black helmet and cape shock. It is not the shock of the scam revealed, it is the shock of the man he truly is being put away, vanished, locked up in the safe, leaving behind the raw elements of what society asks for and accepts, a Saul Goodman.
If you need more evidence… Toward the end of Breaking Bad with the Saul Goodman era over and Jimmy is waiting in the bunk room for his relocation his parting advise to Walter is turn yourself in, face it, best thing for your family.
That is James Morgan McGill.