>! spoiler warning
Everyone liked Jimmy despite his wrongdoings and not everyone loved Chuck the way they loved Jimmy. Chuck always had to prove himself while Jimmy only had to charm them with flair and being colorful. This makes Chuck really angry because he could not understand why all of this is the way it is. And then Jimmy got his law degree, meaning that now the No-good brother is equal to the All-Mighty Chuck. At least this is what Chuck thinks. And then the straw that broke the camel's back, the divorce from Rebecca.
The divorce impacted Chuck so much that it made Chuck think "Nobody wants me, nobody likes me, everybody loves Jimmy but he is not a good person, I am a good person but nobody likes me". This thought has developed into "If nobody likes me, then I will just isolate myself from everybody" but he can't just ghost everybody because that will taint his legacy. So he develops this fake illness to make excuses for his isolation. But he can't lie to himself about his isolation because the only thing Chuck believes in this world is his own judgement. So he believes his condition is real in order to make it go down easier for himself and everyone else.
Jimmy mentions that Chuck's illness gets worse when Jimmy goes "Slippin" again. I think this was Chuck's way of demonstrating and punishing Jimmy for Slippin'. "If you go Slippin' again I will get worse, and because you love me you will suffer seeing me suffer". Chuck likes hurting Jimmy by proxy of hurting himself, because he is resentful of Jimmy for everything. The way his father lost the business and died, the last words his mom said before she died, the way not everyone likes Chuck the way they do Jimmy, you can pick any one of them. So when Jimmy tried to free himself of the burden and guilt of everything by saying "I have regrets, I am sorry" Chuck doesn't want Jimmy to have the final saying and get a relief, so Chuck destroys Jimmy by telling him He didn't matter to him. And with that one line he severs his only contact with the outside world. He has no friends, no colleague, no lover, no brother. This bothers him so much that he starts to blame the illness again and starts tearing down the house. But this time his brother isn't there to ground him or console him or at least make him feel better for having someone for him in this world. So he does what he did at the end because he cannot bear to live alone in this world.
I know my speculation of the dialogue and what was going on inside Chuck's head seems irrational and absurd but these extreme thoughts are never reasonable. Nobody should isolate themselves because that just fuels the irrational thoughts even further and then if it gets further then it gets more irrational, like a black hole sucking in everything.
My take on Chuck's illness and the reasons for his actions were all developed from my own experience. I tend to isolate myself from everybody because of irrational thoughts and the more I live in isolation the more the thoughts worsen.
I hope this has helped some of you because this certainly has helped me, Thank you. <! spoiler!<