The developers never explicitly say that the Thorntons were slavers (for reasons mentioned in the quote source above) but the sheer number of ways they hint at it must be seen to be believed.
There is a room filled with all manner of farming implements, one explicitly made for processing cotton. According to the EMF reader, it is more haunted than any other spot on Blackrock Island. Added to that is the loosely secured scythe that hangs from its rafters, which will kill you if you try to enter Harper's passageway too early.
Most of the Blackrock graveyard houses Thornton corpses, but there's one overgrown tombstone that says simply '54 souls.' This was placed after the Thorntons refused to provide their workers with proper lodgings and heat sources during winter, and, in doing so, accidentally gassed them to death. One is reminded unpleasantly of mass graves.
The fact that we never find out why Clara's father is such a touchy subject with her is enough to raise eyebrows. Harper expecting Clara to cry when you ask her about him is even more suspicious. And thenWord of God refused to say why exactly no one talks about Jackson- but did say that Clara envied Charlotte's "happy childhood." So, yeah.
Danger on Deception Island
The option to poison Katie.
"Say goodnight, Nancy..."
Creature of Kapu Cave
One of the possible death scenes: Nancy fails to make a jump, and falls. Into a pit of lava. From first-person perspective.
Haunting of Castle Malloy
The bar, which Nancy comments is a place to relax "over your favourite combination of fruit juices." The bar which you are asked to mix drinks for. Drinks that are obviously beer.
The Silent Spy
When Moira comes back after her kidnapping, Nancy asks, what was happening to her when she was gone. And Moira says: "Maybe, I don't want to tell. Maybe, it's personal". That... does sound a bit suggestive.