In Chapter 7 of Frankenstein, I think this is the quote you are looking for:
I revolved in my mind the events which I had until now sought to forget: the whole train of my progress towards the creation; the appearance of the work of my own hands alive at my bedside; its departure. Two years had now nearly elapsed since the night on which he first received life; and was this his first crime? Alas! I had turned loose into the world a depraved wretch, whose delight was in carnage and misery; had he not murdered my brother?
And perhaps this one too:
My tale was not one to announce publicly; its astounding horror would be looked upon as madness by the vulgar. Did any one indeed exist, except I, the creator, who would believe, unless his senses convinced him, in the existence of the living monument of presumption and rash ignorance which I had let loose upon the world?
This is a case of metonymy and synecdoche: Victor uses phrases like "rash ignorance" and "the appearance of the work of my own hands" and "my tale" and "its astounding horror" as substitutions for both the monster (his creation) and a part of himself, his dark side or alter-ego.
Indeed the monster is the product of Victor's morbid thoughts, the sum work of his journal, and his own fears. It's as if the monster is an overgrown murderous child, an Id run wild. The monster is indeed Victor's doppelganger, his ghostly twin who has come to haunt his family as revenge for abandoning him and not making him a mate.