The damaging evidence against Justine is a photograph that the creature has planted in her pocket. This photograph belonged to William, the murdered boy. Frankenstein keeps silent about the real murderer because he doesn't think people would believe him. On the contrary, maybe they would lock him up in an asylum, and then his dearest would remain vulnerable and unprotected. In my opinion, Frankenstein wasn't right to keep silent all the time. He was able to prove the truth because there were multiple witnesses who had seen the creature. Creating the creature was not Frankenstein's main sin; failing to cope with it was.
