Hamlin had Chuck "Suicided"
That's right - Hamlin killed Chuck. Chuck was becoming a massive liability to the firm after that last incident and his ongoing condition. Hamlin faked that suicide note for Jimmy as well - that's why it didn't affect Jimmy. Somewhere deep inside Jimmy knew that this is not something Chuck would genuinely say.
Hamlin is now living with the guilt and stress of his murder. Saul will start to figure this out and put all the pieces together, eventually leading to a massive court battle where Saul takes down HHG, proves his prowess as a lawyer, and is ultimately vindicated.