Martha Marcy May Marlene

One of the turning points that made Martha (Elizabeth Olsen) flee the cult, was when she witnessed fellow cult-member Katie stab a homeowner after they were discovered having broken into his home, with Patrick justifying to her that death is a good thing.

Back in the present, things come to a head with Martha's odd behaviour culminating in Ted (Lucy's husband) being attacked by Martha during a nightmare. Lucy states to her sister that they can no longer take care of her and she needs psychiatric help. The next morning as they are driving her to a facility, Martha spots a cult member following them.

Thanks ND Woods!