4 ideas
In Oslo, married mothers vanish on the day of the first snowfall, and a snowman appears at each scene. Detective Harry Hole links the disappearances to a pattern reaching back years, including an unsolved Bergen case, and finds that each victim had a child who was not fathered by her husband. The killer is Mathias Lund-Helgesen, a doctor and the partner of Harry's former lover Rakel, who had placed himself inside the detective's personal life.
The killer does not choose victims at random. He chooses women who carry a specific concealed secret, a child fathered outside the marriage, which his medical access lets him detect through blood and parentage evidence. He frames the murders as punishment of maternal betrayal, a replay of his own origin. The victims look unconnected until investigators find the hidden trait they share.
Look for the adversary among the people the investigator trusts and sees every day, not among outside suspects. A killer who studies his hunter can enter the hunter's intimate life, and closeness then works as camouflage. Familiarity lowers suspicion at exactly the point where scrutiny should be highest.
When investigators or suspects fit a case too neatly, the fit can capture the whole inquiry. The team chases plausible false leads, including a professor under suspicion and a disgraced dead policeman, while a colleague with a hidden family motive pursues her own agenda inside the case. Evidence that feels compelling because it matches a story, or serves a private need, pulls attention away from the actual mechanism of the crimes.