fargo carries a title "based on true events," but it bears little relationship to the woodchipper murder that occurred during a hard Connecticut winter.
Similarly, Texas Chainsaw Massacre claims truth, but the events in the film never occurred in the life of Wisconsin's Ed Gein, the real-life inspiration to Tobe Hooper's film
No, Texas Chainsaw Massacre was loosely based on Ed Gein (and I mean loosely) but most of the idea came from him seeing a wall of tolls (including chainsaws) at the Hardware Store.
Also Infamous, based on the same events as Capote, but with a different angle. Both are stellar, but although based on a terrible crime I wouldn't consider them a part of the horror/thriller genre.
Ok so i just watched Them and it was very similar to The Strangers. Them didn't make me jump as much but i think it was b/c i had already seen the Strangers. I tried to do some digging on the real story of Them but i couldn't find anything other than this:
Them makes the claim to be based on a true story, although one has been unable to find any English-language sources that give details about the incident in question. One suspects that the filmmakers are doing something akin to what The Texas Chain Saw Massacre did, which claimed to be based on fact but had so little resemblance to actual incidents that it had to be considered an almost complete fiction. Even assuming that Them has some basis in fact it must surely be akin to Open Water (2003), which claimed to be based on the true life disappearance of a couple who were abandoned in a sea of sharks. In that both Open Water and Them tell the events that supposedly happened from the perspective of the couple that were killed – ie. failed to survive the events to tell their story – then the bulk of what happens in either film surely has to be entirely speculative. Them has a number of similarities to in particular the Backwoods Brutality film The Evictors (1979) about a couple who rent a farmhouse in rural Louisiana and come under attack by mysterious strangers, which also claimed to be based on a true story.
How did I ever miss The Evictors? I was going to movies at least three times a week back then, especially drive-ins, and watching anything that had Jessica Harper in it.