4 ideas
In 1938 Reinhard Heydrich forces Berlin private detective Bernie Gunther back into the Kripo to hunt a killer of blonde Aryan teenage girls. Gunther finds that SS-connected occultists have engineered the murders to look like Jewish ritual killings, reviving the blood libel so it can justify a pogrom. The investigation runs up to the eve of Kristallnacht, and solving the case cannot stop the violence the case was built to license.
Under a totalitarian regime, law enforcement and organized crime are the same institution, so a detective's real puzzle is not who did it but which faction of the state wants it solved and why. An investigation becomes a tool in an internal power struggle. The truth is useful to those in power only when it damages a rival.
When the law makes a private identity punishable, as Nazi Germany's anti-homosexual Paragraph 175 did, every person with that identity becomes a blackmail target. The victim cannot go to the police without exposing themselves.
A person who wants to do some good inside a corrupt system has to accept its uniform, its rank, and its orders to get access. Each compromise buys a little room to act, but it also binds them more tightly to the regime. The decent insider ends up useful to the very power they are trying to resist.