Here's another idea for a scenario based on a historical event. The setting involved - the United States in the 1920s - inevitably points towards a classic-style Call of Cthulhu story, and the rather grim nature of the events in question inclines this towards the realms of a horror story too.
The Actual History
Prohibition is one of the elements of American life in the 1920s and 30s we are most familiar with, as it is a feature of many of the best-known films and TV shows about this period. Between January 1920 and December 1933, the production, importation, transportation and sale of alcoholic drinks was illegal throughout the United States. Even after national prohibition ended, the ban remained locally in force in some states until 1966. (A 2014 opinion poll suggested nearly 20% of Americans remained in favour of a similar ban on alcohol.)
Over Christmas 1926, hospitals in New York City found themselves hit by a wave of desperately sick people, some of them suffering from bizarre hallucinations. Over that Christmas period, over thirty people died, and many more were made seriously ill. What caused the outbreak? The victims had all been drinking: it was Christmas, after all.
However, the fact that the drinks were lethal was not the result of careless production techniques or some sort of accident. In an attempt to dissuade people from drinking, the US government had poisoned supplies of industrial alcohol, which was used in the production of illicit hooch. This was not kept secret. The official position was that anyone dying as a result of drinking poisoned alcohol had brought it upon themselves. By the end of the prohibition period, at least 10,000 people are estimated to have been killed.
It's a grim footnote to this period of history, and perhaps difficult to believe. It raises many serious questions about the rights of government and the individual, and other aspects of society (the dead disproportionately came from the less prosperous strata of society, who couldn't afford better-quality spirits).
Fight (for) the Power
If you don't want to keep the 1920s USA setting, there's still potential inspiration here for a scenario or campaign about an authoritarian government openly or secretly trying to manipulate the behaviour of its citizens, with potentially deadly consequences. The authority can be as brazen about this as its real-world counterpart, or a string of unexplained deaths can be the first clue that leads player-characters to discover the establishment is conspiring to control them and everyone else.
Or, for a more subversive and morally ambiguous take on this idea, it could be a situation like the one in the Judge Dredd story The Man Who Knew Too Much, where a tanker crash causes thousands of mysterious deaths. A journalist investigates and discovers the tankers are regularly carrying vast quantities of tranquiliser gas, which is routinely inserted into the city's air supply, keeping the population docile and easier for the Judges to control. Judge Dredd justifies this as being essential to keeping his dystopian city governable, and the story ends with the journalist being dragged off to have brain surgery which will wipe his memory of what he's discovered.
The player characters could likewise be enforcers for a secret regime which carries out questionable acts supposedly for the good of the people - they could be agents of the Technocracy in a Mage game, or more akin to the Operative from the movie Serenity. Whether they just revel in their power or come to question the morality of their actions is obviously a choice each group will have to make.
Hooch and Homicide
As noted, the setting naturally leads one to think about handling this story in a Call of Cthulhu scenario. The obvious spin would be to put something in the contaminated alcohol other than simply poison: the published scenario Blackwater Creek features just this idea, with moonshine that's been tainted with the fluids of Shub-Niggurath.
Obviously, there are other options - this isn't a million miles away from the concept of Mythos-contaminated cheese, which we discussed in last week's scenario seed, and many of the same possibilies apply. It's just a question of deciding what's in the booze, how it got there, and why.
Alternatively, you could keep the idea of the deadly alcohol and have the villain deliberately planning to use it to cause the deaths of (largely) innocent people. In a non-Mythos game, he could just be a lunatic, although this isn't the most satisfying idea plot-wise - following clues and figuring out the villain's motivation and identity is the stuff of good play. In a Mythos game, the antagonist could be an adherent of a particularly brutal and bloodthirsty cult, or one seeking to enforce conformity and obedience at any price. Either way, a race against time to stop the next consignment of deadly moonshine from reaching the palates of unsuspecting topers is the logical way to bring this kind of scenario to a conclusion.