4 ideas
Giuseppe Lojacono, a Sicilian inspector falsely named by a Mafia informant as a collaborator, is exiled to a Naples precinct where he is given no real work. A killer begins shooting young people and leaving tear-soaked tissues at each scene, so the press names him 'the Crocodile.' Working informally with magistrate Laura Piras, the sidelined Lojacono sees that the victims are not random, and his case becomes the investigation that restores him.
The most complete revenge on an adult is to kill their child rather than the adult. The target survives to live with a permanent loss, and a killer who does this can grieve the victim, shown by his 'crocodile tears,' while still carrying out the punishment. The child is the instrument, and the actual victim is the parent.
When killings look random, the pattern may lie one step removed from the victims, in who they belong to or what their families did. Investigators who compare the victims' own traits find nothing. The link appears only when you ask whom each death was meant to hurt.
One informant's accusation can wreck a police officer's career and family without any charge or trial, because institutions protect themselves by exiling the accused. The disgraced outsider, with nothing left to protect and no ties to local assumptions, is then the one able to see what the established precinct misses.