The Cabin In The Woods (2011)
Asked to spoil The Cabin in the Woods someone wrote
I finally got around to watching this. "Everybody" means more than just the five teenagers. It ends with every character, from a large cast, save two, dead and the strong implication of end of humanity. It's a rather impressive ending.
(Those two: Guy on roof at beginning and the gas station attendent are not shown to have been killed yet.)
I watched it on DVD and watched the various special features. According to a Q&A with (Producer / Co-author) Wheedon and (Director / Co-author) Goddard stating that every creature shown at the end has a trigger item in the basement and inviting viewers to pause the DVDs to find them. I did not do that. The blood for the image outlines is explained to be generic sacraficial blood and not from the teenagers by Wheedon.
Well cast, nicely plays with the tropes of the genre, and finds a deeper unifying story. Although never answered (in film or in Q&A) is how the agency got started and how they know what they know about the ancient monsters.
Also "Apparently", the original release date was supposed to be about a month after the premiere of Tucker & Dale vs Evil, which is another good humorous take on dead-teenagers-in-a-backwoods-cabin film.
T & D at wikipedia The film premiered on 22 January 2010 at Sundance Film Festival
Cabin at wikipedia The Cabin in the Woods was slated for wide release on February 5, 2010
Final thought: and why have a "purge" button?