One of the particularly interesting aspects of Frankenstein is its exploration of free will and determinism. This debate, in broad terms, is the question of whether we are genuinely able to choose how we act, or if our actions are instead predetermined, entirely outside of our control. This is something that has been discussed by philosophers since ancient Greece; Shelley's novel uses science fiction to bring the debate into a modern context.
What makes the matter of free will and determinism particularly salient in Frankenstein is the fact that, by stipulation of the part of Frankenstein, the creation of his monster was entirely determined by him: he uncovered the precise scientific procedure underpinning each step up to the monster's creation. Such a procedural design of a living being makes it easy for us to draw the inference that the monster, as an entity, has been entirely architected by Frankenstein -- and, crucially, the monster is not ultimately responsible for his actions, since he could not choose otherwise.
It is true, of course, that the monster learns things after his creation; yet the story suggests that this learning process only further served to determine the behavior of the monster exogenously. Namely, the monster says that he learned how to feel and think about things through books -- Paradise Lost, Plutarch's Lives, and the Sorrows of Werter. So while Frankenstein may not be wholly responsible for the actions of his monster, it still does not seem that the monster is responsible for his actions.
This is another way in which the monster is made to seem conceptually distant from humanity: because the reader has knowledge of the invention of the monster, we can easily suppose his actions to be determined, and thereby view him much more as a high-functioning automaton than an extremely ugly human. Yet the novel makes this issue more complicated: though the monster may seem distinct from humanity for this reason, is this actually something that distinguishes him from humanity?
The parallelism at work in the novel suggests that humans may function in the same way as the monster. Both Frankenstein and the monster account for their actions by describing their origins and the different external forces that influenced them; the only difference is that we explicitly know that Frankenstein constructed the monster. Yet if we stop and consider Frankenstein, we know that he, too, was created: he was merely created by two parents instead of a single scientist. Is that difference conceptually significant? Perhaps not: in both cases, the science by which the organism was created largely determines, in conjunction with external influences, how the organism will behave.
This is essentially a thesis of behaviorism: the notion that people's actions are programmed based on a combination of their biology and environmental feedback. Nowadays, such a view is considered reductive with regards to human nature; but what Frankenstein importantly underscores is that there is at least some degree to which our actions are not conceived purely as a function of our own agency. People, literature, and our environment in general prompt us to act and think in particular ways -- and perhaps, as such, we ought to broaden our scope of precisely who and what is culpable for the actions of any one person.