Why did Justine confess and what was the result?

Justine, called to the witness stand, provides another account of the events: with Elizabeth's permission, she had passed the night of the murder at her aunt's house in Chêne. Upon hearing of William's disappearance, she spent several hours searching for him; unable to return home, as it had grown too late, she determined to spend the night in a nearby barn. Justine says that if she was near the body, she did not know it; her confusion was only a manifestation of her tiredness. She remains unable to explain how the picture came to be on her person; she can only assume that the murderer himself placed it there.