Both of these fine films deal with that seamiest side of law enforcement--the undercover cop. And both are true stories.
In "Donnie Brasco" an FBI agent infiltrates the New York underworld, primarily by befriending a mid-level guy played by Al Pacino. We see their friendship develop and all the moral ambiguities that present themselves to someone leading this sort of life. The guy's family life goes to pot, of course.
In the most telling scene in this film, the cop is in Miami working on a business venture there, when he meets with the other FBI folks in a makeshift headquarters set up in what appears to be a motel room. The "suits" are listening to wiretaps, and generally just pushing paper around, while the undercover guy, who is the only one dong real work, and certainly the only one doing dangerous work, just does not seem to fit in. He has taken on the personality of his alter ego, Donnie Brasco, and by this point he prefers the company of his friends who know him as Donnie Brasco.
The callousness of the FBI toward Donnie and his precarious situation is appalling. He is used as a piece of meat for their own ends, and this is troubling. The last scene is especially troubling, as the agent is presented with a plaque and a paltry $500 check. Hardly anyone is there except for his family, and it is obvious there is no real appreciation for what he has gone through.
In "Prince of the City" a cop who is disgusted with what he has to do in his undercover detective work decides to work with the federal authorities who are investigating police corruption. The real-life result, as mentioned in the bonus part of the DVD, is that 52 of the 70 members of New York's Special Investigations Unit (SIU), are indicted on criminal charges.
At the start the feds ask him if he has done anything improper, as he will be asked about this on the stand when he eventually has to testify. He mentions 3 things, but of course there are many more and the most telling scene in this film is a huge meeting in the Attorney General's office with all the prosecutors there giving their opinions to the AG on whether to seek perjury charges against the poor guy. Passions run high on both sides, and after hearing all sides the AG decides not to prosecute him. On the same day, ironically, the Judge in a Motion for a new trial against the highest-profile defendant, a crooked lawyer, denies the new trial request, which was based on all the false testimony of the detective, holding, correctly I think, that it was a "collateral matter" and didn't affect the facts of the case against this defendant.
The SIU group is presented as basically a very dedicated group of cops, who do take some money and take some dope on occasion, but have also taken plenty of bad guys off the street. As in "Donnie Brasco", the feds are not presented in a very good light. The 2 prosecutors who worked most closely with the main character are decent guys, but some of the higher-ups are just pricks. In fact, a scene which the audience applauded in the theater had one of the cops charging into the office of this particular prick prosecutor and assaulting him. This cop is later presented as the only true hero of the film, as he is going to fight the charges rather than cooperate with the feds in exchange for leniency, or commit suicide, which are the options chosen by all the others.
These films present troubling pictures of the use of undercover informants in our society. Justice Douglas went so far as to say he thought a free society should never use undercover informants. I don't know that I would go that far, but certainly their use should be quite limited.
Part of the issue here is that the types of "crimes" these guys usually work on should arguably not be crimes at all--things like use of dope, prostitution, and gambling. If we can get these things de-criminalized, that would go a long way to ridding our society of the stench of undercover informants.