Wiseguys are mostly working class Italians who just happen to be employed as criminals. They have girlfriends, wives and children; they complain about the bills, they enjoy a good game of cards, they eat out and socialize; and they murder people without thinking about it the way we would use the copier at work. Brasco said the stereotype is that they're all hard partying drinkers. Some of them are like that, but a surprising number of them are teetotalers. Adultery was commonplace, but never with another wiseguy's girl. Disrespecting the wife or girlfriend of a member could get a man killed.
The lower ranking members were perennially broke as a joke. Brasco's Mafia partner, Benjamin "Lefty Two Guns" Ruggiero, was always on the lookout for the next big score. He was never off. Wiseguys made it a point of pride to spend as little of their own money as humanly possible but they still managed to rack up impressive debts. Lefty unwittingly caused enormous headaches for the FBI by insisting that Brasco and other undercover agents pay for his rental cars, hotel rooms, and restaurant tabs.
Brasco had to walk a fine line. As an FBI agent he could not commit crimes; any stolen goods he handled were turned over to the Bureau. But he had to create a reputation for being a stone cold criminal. Only a handful of good guys knew what he was doing. As the operation went on, his life was in danger both from the cops and from the bad guys. If he learned of a Mafia plot to murder a civilian, he was obligated to stop it. But he decided early on that if he was involved in a hit on another criminal, if it came down to himself or the bad guy, the bad guy would die.
Beneath all of the Italian bravado, the driving emotion of Mafia life is fear. Associates feared soldiers. Soldiers feared their captains. The captains feared the bosses. The bosses feared each other. Establishing gambling, loan sharking, and racketeering operations required a lot of money. Wiseguys are judged first and foremost by how much they "earn." Soldiers had to share the profits with the captains, and captains had to share with the bosses. Holding out on your fellow wiseguys could get you killed if the numbers were big enough, but even so it was a common practice. Never spend your own money, and keep as much as you can get away with.
Despite their ruthless approach to killing, the Mafia generally doesn't retaliate against cops or judges as doing so would bring down too much heat. When Brasco was revealed to be an FBI agent, they made an exception by placing an open $500,000 contract on his head. The Bureau informed them to drop it or else the entire Justice system would make it a point to wipe them out. Nearly every wiseguy who so much as had drinks with Brasco was whacked.
The saying goes that there's no honor among thieves, but the Mafia has its own code of honor. Brasco's Mafia captain, Dominick "Sonny Black" Napolitano, said he harbored no ill will against Brasco for his deception. He was an agent who was doing his job. Sonny's body was found in a shallow grave with both hands cut off, a symbol of someone who compromises the family's security. Lefty was quickly apprehended by the FBI for his own protection. Brasco, for his part, harbored no ill will against his Mafia partners. He ended the book by saying professionally he'd do it all over again, but personally, it created too great a strain on his marriage.
If you're at all interested in criminology, read this book.