I like Jimmy. He and Richard create some space for war semi-real talk, and it's
that he feels like he's got nothing more to offer the world than being a killer after serving in World War I.
His unraveling throughout season 2 was... hard to really understand at times, but the deck was stacked against him (I lol'd irl at him reading Jacobean theatre), and his senselessness is I guess a personalization of the senselessness of World War I and Prohibition.
As for the rumors described in the spoilers, I can kind of see it with how hurried the megatons were dropped about his past in the final episodes, but really he was unraveling from the beginning of the show. Like he said, he died in the trenches.
Wish Angela got a better run, underutilized character imo.