After watching You're Next, I thought about this idea for a horror film that changes things up after the first act ends (much like You're Next and From Dusk Till Dawn). It starts out with the main character attending a dinner party in a large house that far from a nearby town. A group of burglars invade the home, cutting off all communication and surveillances as they take the guests and their hosts hostage as they rob the place. The mansion goes into lockdown, which means the thieves cannot escape and then shit gets real when the second act starts. Hostages and burglars start getting picked off, one by one as fear and paranoia spreads. The lead thief made a point before the break-in not to kill anyone and suspects his second-in-command is trying to take over.
Eventually, the only ones remaining are the main character, the leader, and the second-in-command. When it looks like the second-in-command was the one killing everybody and is about to defeat his fellow thief, a big twist is revealed. The main character seemingly saves his captor only to reveal he is the actual killer who planned on killing those at the party but modified things when the burglary took place. He carefully made sure there was no evidence that pointed to him while helping to sow the distrust by leaving clues that would frame both thieves. The film ends in one of two ways. Either the main character kills the last thief, make it look like he was never a guest at the party, and escapes, leaving what happened as an unsolved case for the police. Or the second ending is the main character still destroy any evidence he was there and escapes but leaves the thief alive. He does this because the thief will be in trouble for the home invasion and will have a hard time convincing the cops that a psychopath was responsible for the murders.
It is probably an interesting idea on paper but would be hard to pull off.