As gamers we have done the western with laser pistols, six shooters and steam tech. Shadow run alone has covered more noir ground than MGM did between 1945 and 1955. I hope we managed the horror genre well, and the murder mystery almost always sneaks its way into our campaigns. But do we always know we are doing it? Can genre sit right in front of us an yet go undefined? I think it has happened and a speculative fiction genre of all things.
A vicious murder happens in a town that the party happens to pop in at. Under heavy suspicion as they are strangers, the party is forced to discover the perpetrator or have their reputations blackened, especially as more and more murders occur in the town, and mysteriously stop and restart when the party leaves to go kill off that evil necromancer who kidnapped the princess.
The only problem is that a demon, possessing one of the party, is the perpetrator. And the demon makes no signs that its living in the PC's head.
In fact, for all the party knows (except for the possessed person's), their companion is an evil murderer.
Do they try and execute their friend as he's a vicious murderer, and no evidence points to demonic posession? Or do they flee town with him, trusting him, and have their reputations destroyed?