There's a part where he sets up a trust so that his family will money the next time he goes bust (which happens constantly in the book), and tells his wife that he will beg and plead for the money but she has to refuse him. That feels to me like the behavior of an addict capitalizing on a moment of lucidity to protect loved ones from their addiction.
The real Jesse Livermore died penniless by suicide. The book doesn't address his depression, but I think you do see it in what's not in the book. They don't really talk about the character's friends. They don't seem preoccupied with their wife or their children.
a market maker (MM) (citadel, optiver) etc makes money in the most part by filtering a trade from customer, and then agreeing or disagreeing with its sentiment. If you buy say msft at $480 and they think it will dip below $480 they will 'hold risk' and wait for it to dip to say $470 then execute your trade, making $10
except instead of aiming to make 2% on a single trade, they aim to make maybe 5 cents this way on a $480 stock, mostly holding for less than a few seconds, on millions of trades an hour
thus, robinhood very valuable because it encouraged retail investors to have high trading volume on complex instruments. that "order flow" then sold to MM like above for fixed rates or % shares.
MM are ultimately a good thing because they provide liquidity. there is always a person willing to take the other side of a trade when you click. if you trade in a boondocks stock market like singapore or new zealand you will quickly see the market run dry and you cant get out of a position