First, just the way it starts: "I hope you will take my words in the spirit in which they are intended." That's universally understood code for 'I am about to **** on you.'
Then there's the meat of the letter, which is basically Chuck saying how proud he is of Jimmy to work in the MAIL ROOM. "You took the opportunity I gave you, and ran with it." He saw Jimmy as his pet that he gave a handout to & kept in the basement, but he never wanted him to be any more than that, never respected his aspirations to be a lawyer. (He admits this flat-out to Jimmy's face later, in an outburst.)
"I'm certain no matter what the future might bring, you'll land on your feet." It reads more like an employment termination notice than final words from a loving brother.
The real key giveaway in the letter to me though, the real insight into Chuck's resentment towards Jimmy, is a throwaway line near the beginning of the letter: "I can honestly say I never saw mom happier than the day *you* were born. You brought a shine to her life that nothing else ever did, and I'm glad of that."
That's a reference to arguably the main source of Chuck's resentment toward Jimmy: his own jealousy, that their mom (& dad) loved Jimmy more (or Chuck thought so anyway). Back in the finale of season 2, they show the flashback where Chuck and Jimmy are at their mom's deathbed in the hospital. Jimmy goes out to get food, and while Chuck is sitting there alone with her, she suddenly wakes up for a brief moment, calling out Jimmy's name repeatedly, then dies. Jimmy comes back later with the food, is shocked to learn she died, then asks Chuck "Did she say anything?" Chuck says "No."
Arguably the worst thing Chuck has ever done to Jimmy, and to me, the defining moment that sums up what a piece of **** Chuck always was to Jimmy.
