A particular 1920s 'gangster' I have researched is Johnny Torrio and his 'mentor' relationship with Al Capone. Torrio was the man responsible for Capone's move from Brooklyn to Chicago, as Capone needed to dodge a murder investigation, and was also the man who hired Capone to a bar in Coney island that was owned by a renowned mobster.
Once in Chicago, Torrio put Capone to work as a bouncer at one of his brothels, and a year later when prohibition was implemented, he had his associate Giacomo 'Big Jim' Colosimo (who was against venturing into bootlegging and perfectly happy being a plain old pimp) killed (possibly by Capone, or some bloke called Frankie Yale, who was Capone's original boss in New York, and is the renowned mobster mentioned earlier), took over his criminal empire and began bootlegging himself, with Capone's help. Colosimo was, at one point, married to Torrio's Aunt, and was being blackmailed by a gang calling themselves 'The Black Hand', so at his Aunt's request, Torrio arranged to pay off the blackmailers, then murdered them all when they showed up to collect. Wonderful. However, shortly before his assassination, Colosimo unceremoniously dumped Torrio's Aunt in favor of a young actress, so Torrio probably had no real qualms with having the horny tubster whacked. There's a lot more, but it's relatively inconsequential and I'm not trying to write a Wikipedia article here.
I plan to use this relationship as inspiration for my protagonist's descent into crime.
I will start with my protagonist working a legitimate job for a certain 'Torrio-esq' individual at the start of the prohibition, who will be an entrepreneurial mobster and owner of a speakeasy, then the protagonist will be offered the chance to bootleg, which he accepts. I will not be basing my protagonist too much on Al Capone, definitely not in looks, as Capone was a fat ugly bastard, and i plan to make my character a lot less of a raging psychopath than Capone was, mostly because there wouldn't be enough people in the cast for him to kill, plus I intend for him to survive the events, as he is the narrator, and that would be very unlikely if he was a trigger happy nutter who shoots everyone in the face.
This relationship is also partly inspired by two characters from 'Boardwalk Empire' - Nucky Thompson and Jimmy Darmody, with a few differences, as in the show Nucky wants Jimmy to avoid a life of crime, whereas my protagonist will be encouraged to indulge himself in heinously illegal activities at others expense.