I believe Kids now a days are corrupted by TV, Internet, Irresponsible Adults an do not have a darn clue of the ramifications of there actions.
She was clueless IMHO that he would kill her parents.
I raised my son by myself for awhile before I had married again, he never gave me one ounce of grief he did not have a computer, he was totally into sports and he knew without a doubt if he gave me any trouble, and if the basebsll bat did not work he would be out on his ear in a New York minute.
If she was that clueless why did she run after his car as he was driving away and tell him she wanted to go with him and get married.... she would have to step over her fathers' dead body to get out the door, so certainly she knew they had just been murdered. At 14 you know the difference between right and wrong.