Nobody gets prickly if you say "I ran over the pedant in my Ford" instead of "I ran over the pedant in my Ford-brand automobile". Likewise, it is perfectly grammatically and logically correct to say "Frankenstein disemboweled the pedant" instead of "Frankenstein's monster disemboweled the pedant".

"Frankenstein" is a synecdoche for "Frankenstein's monster" (or "a Frankenstein-brand monster") the same way "Ford" is a synecdoche for "Ford-brand automobile."