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I have an idea.
The setting is modern, and the location is some kind of castle/mansion/estate/villa, etc.
The basis for the city is a very influential criminal group, like an Italian mafia family. Among them is a small group of conspirators and one avenger (a wife/brother/son/daughter/father/mother) whose family members were once killed by the group.
I want to focus on the mechanics of the murders. The idea is to give the "mafia" or the serial killer more opportunities to plan and carry out murders. For example, choosing weapons, perhaps the number of strikes, and staging the murder as a whole. The main limitation will be the specific role, which I plan to make quite broad. The "GM clue" will probably be in the form of some kind of mistake made by the killer, a slip-up. But this means it will be obvious.
Therefore, all "perks" for the killers are immediately excluded, as they will be unintentionally present in every crime.
That's the general idea.