The Creature in Shelley's Frankenstein is not nurtured and, thus, becomes a Monster.
Victor rejects his creation immediately due to the Creature's appearance. His rejection is due to the superficial. The creature, left alone to progress on his own, becomes hideous. This is what becomes of natural man.
Contrast this, for instance, with Elizabeth. She, too, is orphaned (as the Creature is, for all practical purposes), but she is accepted and loved and taken care of. And she becomes an almost ideal person.
Victor bears responsibility for the Creature becoming a Monster, because he fails to nurture his creation.
In order to analyze the theme of nature versus nurture in Frankenstein, consider the conditions under which the monster becomes distraught and destructive. On the one hand, Victor believes that the monster may be inherently evil, and he thinks that the monster's destructive nature has come from the manner in which Victor created him. However, Victor has shunned the monster; and as a result, the monster feels lonely and rejected by his creator. So, is the monster evil by nature of creation, or has he been nurtured (or not nurtured enough) by Victor to act in these evil ways? A question like this might be posed to a group of learners to allow them to explore the possibilities of this theme in Frankenstein.