When I first read a book, I look to the title. The title of a novel, more often than not, is a big, glaringly obvious clue to the entire book. In fact, the characters, destinations, dialogue, it's all periphery to the title.
Mary Shelley at 18 years old wrote a book that helped Hollywood churn a mass of movies and inspired many books. At 18 years of age Shelley was able to write a book that Professors of Literature pored over, spilling ink and blood to enunciate every fine detail; even though many still confuse Frankenstein as being the name of the monster.
But, when a pedant cries out Frankenstein is the Doctor, not the monster, can we not reply with a wry smile and a knowing wink, with a nod to Shelley for littering the clues in plain sight, possibly the best place to hide something according to Poe.
In this epistolary novel, Frankenstein 'creates' a monster, a Daemon, something he grows to hate intensely, something he bitterly regrets and wants to extinguish from his life. This 'creation' is realistically not feasibly recreated from bits of dead bodies dug from fresh graves, stitched together and given life by a Promethean creator. Frankenstein is the monster and vice versa, and Mary Shelley tells us this constantly throughout the novel; maybe not implicitly as in the case of Dr Jekyll and Mr Hyde, but certainly the clues are there, hidden in plain sight.
Like Dr Jekyll, Frankenstein is a scientist, perhaps mad, that locks himself away in solitude, in his laboratory, working on experiments that mainstream medicine would deem morally and legally reprehensible.
Dr Jekyll's dark side came about through chemical process, the 'wolfman' a biological process. The 'Invisible Man' allowed his wickedness free reign because no one could see the real man. Frankenstein is a serial killer. He suffers an 'addiction' to killing. He fits the profile perfectly and Shelley lays it all out for us.
When the Irish prepare to give Frankenstein a death sentence, he confesses his crimes in a fevered state, luckily for Frankenstein, the Irish do not understand his language and his doting father provides an alibi for the murder. Frankenstein even asks why he did not simply die from the fever so this would all be over, his compulsion will not release him so easily.
Frankenstein, being a man of science from a respectable family suffers cognitive dissonance. He cannot reconcile being a 'good man' whilst having these compulsions to kill. His necrophilia was the first clue we were given in the novel, it's not that much of a leap to see the connection, in the same way a man that steals ladies underwear from the washing line, if not caught, needs a bigger thrill and ends up raping and murdering a woman.
We see again when the monster asks Frankenstein to create a 'mate' so they can be happy. Torn, Frankenstein at first agrees, then later changes his mind and destroys the incarnation that was being formed. Seemingly one monster in his head is more than enough to cope with, never mind two.
The 'monster' Frankenstein keeps referring to is the monster he can't control. His urges are too great. Like an alcoholic, drug addict or gambler. Sometimes the monster takes over and, as in the case of Frankenstein, the urge to kill is great.
When the monster talks of turning up on Frankenstein's wedding night and Frankenstein's feels relieved that finally, either he or the monster will perish, it is of course Frankenstein's wife who is killed, not by some extraneous creature, rather by the hands of Frankenstein himself.
There is a sort of tragi-comic theme running through the novel, a biblical pale rider where death follows where Frankenstein treads and deep down his friends and family know that Frankenstein is a sick man, the monster, but they cannot hold that thought, so denial kicks in and consequences follow.
Shelley may not have realised the reach this fireside horror story would have, but it has kept people reading and will undoubtedly continue to do so.