In Frankenstein who was accused of committing the murder and why?

Justine Moritz was accused of committing the murder of William. She soon confessed to the murder (though the monster Frankenstein actually murdered William) and was executed 1 day after. The police found Justine as a suspect because William had what appeared to be a thumbprint or a mark of her locket that she kept with her on his throat. We know now that the mark was probably was delivered when Frankenstein was murdering William. I say murder, but Frankenstein really didn't know any better.

Hope this helps