I thought the actress who played Terra played that scene perfectly. Her survival instinct kicked in when John attacked her and she did everything that was necessary to Not be a victim. When she goes into shock afterwards, thinking she'd killed him, man that gave me chills. I haven't seen anything like this in real life, but that is exactly how I would expect a person in Terra's position would act.
God I hope that Debra and her daughters are doing therapy, both group and individual.