My take on the film (spoiler)
The characters Beth and Anna are reversed and it is not until after Anna dies are we the viewer shown this. When Beth wakes the next day we see her dressing in Anna's attire and seeing Anna work out on the porch with same attire that Beth had previous worn. It's here that I believe that it was really Anna who was the sassy character and Beth the more reserved. Beth projecting her ambitions of wanting to be more like Anna made us the the viewer see her like this. With Anna dead the illusion is dropped.
It was really the sassy Anna who attacked Beth that night concluding with Beth killing her in self defence/retaliation. We see this when Beth enters the woods and remembers the events of what occurred.
Any thoughts?