5 ideas
The solution rarely satisfies him: the culprit is often more sympathetic than the victim or than the system that will punish them. He makes the arrest anyway and is left with the melancholy of the title, so each story ends not in triumph but in a competent man's quiet grief at what his competence served.
A rule-bound popular genre like the puzzle mystery reads as apolitical entertainment, so censors let it through. Inside that frame an author can show informers, careerists, cramped housing and small hypocrisies as mere background texture for the plot. The critique sits in the setting and the mood rather than in any stated argument, which lets it survive scrutiny.
Following the method perfectly can yield a true answer and still a wrong outcome, because truth-finding and justice are separate operations. In a state where the law serves the regime, the honest investigator becomes an instrument whose accuracy can deepen injustice. Borůvka's sadness is the felt gap between being right and doing right.
Borůvka is honest, humane and good at his job, yet he works as a loyal part of a repressive state's police apparatus. His personal decency does not redeem the institution. Instead it makes the institution function better, and his melancholy is the private cost of that complicity, which he never turns into rebellion.
The fair-play detective story promises the reader that every clue needed for the solution is shown openly and that logic will reveal what really happened. Placing that contract of transparent, checkable reasoning inside a society built on official lies and hidden motives turns the form itself into a contrast. It shows a world where truth is findable and then asks what that truth is worth there.