She is as misinformed as you are...that's all.guilty. What does it say about a man whose only living child has nothing to do with him and believes he is guilty? Don't suggest that her mother turned her against him either because she's an adult and old enough to make her own decisions.
Mark Twain - "If you don't read the newspaper, you're uninformed. If you read the newspaper, you're mis-informed."
Same with Kim's parents. Plus, they have the added pressure to believe that David is guilty because until Boney was uncovered, which if David knew about him don't you think he would have ratted him out prior to being sentenced to THREE LIFE SENTENCES!!! But I digress, before they new about Boney they HAD to convince themselves that David was a monster and could do this...once they believed that, nothing will change their minds.