Dark Ride
(2006)

Only Cathy (Jamie Lynn Sigler), Bill (Patrick Renna), and Jim (Alex Solowitz) survive the night.  While Bill was telling the truth about his cousins he left out one small detail; the killer Jonah is his older brother.  Bill was not repulsed or shocked at his brother's crimes but entranced since he believed Jonah did them for him.  Bill stabs Jim to death just around the time Cathy wakes up from crashing the van into the fun house.  Bill thanks her for her help (Cathy had brought the gang for a revenge against her ex, but Bill actually wanted them there for Jonah to have some fun) as he looks at his dead brother.  Cathy goes outside and breaks down as the police arrive.

The film ends on an ambiguous note as we wonder if Bill will kill Cathy, or even if he will share any culpability in his brother's crimes and the killing of Jim.

Thanks Spectre!