The Twist: Throughout the film, we believe the gangsters are coming to kill Grace. In the climax, it is revealed they are her father's men, coming to retrieve her. The "Big Man" is her father.
The Ending: Grace and her father have a conversation about arrogance. Grace realizes that by forgiving the townspeople, she has been treating them with condescension. To be truly moral, she must judge them as she would judge herself. She exits the car and orders the gangsters to burn the town and kill everyone. She personally executes Tom Edison Jr. The only survivor is Moses, the dog, because he was the only one who was honest about his nature (he barked at her).