Three games in, Luigi's Mansion is an action-adventure series with mystery and light horror elements, making it a paradigm example of the Nintendo Treatment. Although the games aren't particularly dark, the series has possessed some fans of a particularly dark theory concerning the mansion's Portrait Ghosts.
In Luigi's Mansion, players control the titular character, who has won a mansion in a contest he didn't enter. After promising to meet up with Mario there, but not seeing his brother anywhere, Luigi ventures into the spooky, old house and attempts to track Mario down. He encounters several ghosts along the way and fights them using a special ghost-hunting vacuum designed by scientist Professor E. Gadd.
One fan theory suggests there is a behind the scenes plot that is often overlooked for the main story. It involves the spooky portraits of people who have long been dead -- Portrait Ghosts -- and the ghost of the mansion's butler, Shivers. The theory posits a classic mystery trope, with a twist: The butler did it.
In Luigi's Mansion, there are a total of 23 Portrait Ghosts, each with the appearance of an undead person wearing the clothes they wore at their time of death.
Professor E. Gadd says the spirits of the portraits where released from their paintings by Boolossus under the command of King Boo. These include Shivers, who wanders the mansion in search of the master's will, hoping he was in it. Shivers is also smitten with Melody Pianissima, but she does not seem to return the feeling.
The Reddit theory, posted by username CrustCrusty, suggests that Shivers poisoned the members of the mansion's family in order to be the sole inheritor of the master's will before dying of old age at 72. As the mansion's butler, one of the responsibilities Shivers had was serving or cooking food.
Interestingly enough, Portrait Ghosts drop a Poison Mushroom item with a bite mark -- and they are the only ghosts who do. There is an unused animation for the game in which Luigi touches a Poison Mushroom and coughs, taking on the same ghastly appearance as the Portrait Ghosts.
Plus, the classic Mario mushroom is based off of a real-life poisonous mushroom -- the Amanita Muscaria -- which can cause hallucinations, drowsiness and loss of equilibrium when ingested. In some cases, people have died.
Many of the Portrait Ghosts reveal what they were doing just before they died. For example, Neville yawns while reading in a night robe. Sue Pea's bio suggests she took a nap and died in her sleep. These symptoms of the ghosts further support the idea that the residents were poisoned.
However, this Luigi's Mansion theory also posits that Shivers poisoned himself -- which doesn't fit with how he sets himself on fire with his candelabra before his death, unless he did so due to loss of equilibrium. It's hard to imagine that the 72 year-old butler suddenly forgot how to handle a candelabra. This makes sense, as realistically, poison affects individuals differently based on things like age, size, health and exposure.
It's also possible he did not poison himself and the mushroom he drops is someone else's. After all, the item can be found by other means throughout the game, like from opening drawers.
In addition to possibly poisoning himself, the theory also suggests Shivers poisoned Melody Pianissima, the subject of his unrequited affections. Perhaps he killed her for turning him down, but that seems unlikely given how smitten he still is in the afterlife.
These holes are not explained in CrustCrusty's Reddit theory, but it does have some merit. Shivers has the motive to poison the others in Luigi's Mansion, since he seems to be the only Portrait Ghost interested in finding the will. Plus, as the butler, he had the means to do so.