Frankenstein 2

"I am only left to conjecture concerning the probabilities by which it might have been placed in my pocket." pg. 56

As far as I have read, there are two suspects for the murder of William. They are the creature and Justine. Justine in the quote above claims that the only explanation to her being in possession of the locket is that someone framed her. If this is true, the creature being the only other suspect, must have framed her. If it is not true, Justine is a very gifted liar and Victor's family is a poor judge of character. If the creature did frame Justine, what was his motive for doing so? Was it to prevent himself from getting caught? But I don't see how this applies because Victor is the only one that knows of the creatures existence, so why would he feel the need to frame Justine? And why Justine? Did they possibly have some sort of encounter?