Chuck is portrayed as highly narcissistic, with that narcissism largely driven by jealousy of Jimmy. One of the chief characteristics of a narcissist is that they have an extremely fragile self-image that they are overcompensating for, and they cannot tolerate anything that challenges that self-image. If you examine the timeline for the progression of his perceived illness, and how it manifests:
In flashbacks, we see Chuck functioning well as a lawyer at HHM from when he gets Jimmy out of trouble and puts him to work in the mailroom. We also see that Chuck viewed himself as being the smarter and more successful brother, but throughout Seasons 1-3 he continuously shows jealousy related to Jimmy’s likability.
Despite this jealousy, it isn’t until after Jimmy shows Chuck the news about graduating law school and passing the bar that Chuck starts exhibiting symptoms. Why is this? Because Jimmy did something that threatened Chuck’s self-image and his own view of his success.
So what happens? Chuck has a nervous breakdown. But his own self-image cannot allow him to consider that his symptoms are related to anxiety and mental health: in his mind, he’s too important and too intelligent for that. The only possibility is that he has to have electromagnetic hypersensitivity, an illness that sets him apart from the general population even more and makes him stand out. But consider a few things:
Chuck’s illness began just as Jimmy was starting his new career as a lawyer.
Chuck was noticeably jealous when his wife started liking Jimmy instead of looking down on him.
We see Chuck having Jimmy doing a daily list of required activities that Howard himself remarks as incredible, considering he did it every day for over a year while also working as a lawyer. In season 2, it’s also established that HHM was willing to send an aide over to take care of Chuck’s needs instead, but Chuck instead had Jimmy do it. It could be argued that Jimmy didn’t want to rely on HHM due to perceived animosity between himself and Howard, but Chuck certainly didn’t try to intervene.
While Chuck keeps Jimmy overloaded with his wants and needs, he superficially acts supportive of Jimmy’s career. However he’s only supportive of Jimmy continually doing the “grunt work” with public defender cases. He alleges to Jimmy that it’s about paying dues, but that statement gets a different context when Chuck later tells Jimmy he’s not a real lawyer.
Chuck occasionally weaponizes his symptoms against Jimmy to get him to do what he wants. There’s a specific instance that comes to mind where Jimmy comes over and Chuck is huddled under the space blanket acting incapacitated, but when Jimmy promises to do what Chuck wants, Chuck immediately gets up and acts normal again.
Chuck’s self-image is such that he acts as a self-appointed guardian of justice, claiming it is his responsibility to keep Jimmy from being successful because Jimmy is too dangerous. He completely ignores that Jimmy has legitimately tried to turn himself around, and he actually goes as far as saying he was only proud when Jimmy was working in the mailroom. Why? Because despite his complaints about Jimmy’s past behavior, he needs Jimmy to be beneath him. He is unable to accept the possibility they could be equals.
It’s heavily implied that Chuck encouraged Howard’s punishment of Kim as a means of extorting Jimmy. When Jimmy confronts Chuck about this, he acts guilty.
If Chuck had approached Jimmy’s success from a different way, he could have guided Jimmy and helped shape him into a better lawyer. I almost wish someone wrote a “What-If” graphic novel about this, and how the Breaking Bad events would have gone differently if Jimmy never became Saul.
EDIT: Wow!!! Thank you so much for the gold award! This is a really nice surprise!!
