It's Elementary dear Costello!

"His jaws opened, and he muttered some inarticulate sounds, while a grin wrinkled his cheeks.  He might have spoken, but I did not hear; one hand was stretched out, seemingly to detain me, but I escaped, and rush downstairs" (pg 35-6).

"About five in the morning I discovered my lovely boy, whom the night before I had seen blooming and active in health, stretched on the grass livid and motionless: the print of the murderer's finger was on his neck" (pg 47).

This blog is not really rooted in any real literary device, but I wanted to play detective so we will just have to see how this goes!  As of right now I am about 72.3% confident that the creation is the murderer.  To start, here is a clip from Of Mice and Men that basically sums up how I see this whole interaction between the creature and William going.

I know this may surprise you, but I do not know them.

In the first quote, we see the creature reaching for Frankenstein much like a baby, and where do babies reach for Costello?  The face.  We recently learned that the creature would kill Frankenstein's "remaining friends," but even in the creature did not know that William was Frankenstein's friend, the two would have had a family resemblance or the picture on the locket would have as well I suppose.  Now, Frankenstein is clearly pretty smart, but he also probably knew that after the murder, he either needed to remove the evidence or give the locket to someone who could get some use out of it.  Another hypothesis is that Frankenstein had been stalking the family in a shed next door and seen the locket on Elizabeth and thought it was stolen, but how likely is that to happen?