Victor is a mortal, but he suffers from extreme hubris (or pride) so much so that he believes that he can--and should--create human beings in a manner that defies nature and natural laws. He says,
Life and death appeared to me ideal bounds, which I should first break through, and pour a torrent of light into our dark world. A new species would bless me as its creator and source; many happy and excellent natures would owe their being to me. No father could claim the gratitude of his child so completely as I should deserve theirs. Pursuing these reflections, I thought that if I could bestow animation upon lifeless matter, I might in process of time (although I now found it impossible) renew life where death had apparently devoted the body to corruption.
Life and death, of course, are not imaginary boundaries for humans, at least not yet (and certainly not two hundred years ago). Victor believes that he can bring "light into [the] dark world" as a god would or could do. He thinks that he will create a new species and be more of a father than any other father has been before (so, more than the god or gods that created humanity). He believes that he can reanimate the dead pieces of others' bodies and create new life. Now, in Greek mythology, any mortal who is so proud that they try to take their place with the gods or usurp their power or challenge them, basically always dies tragically. Phaethon wants to drive the sun god's chariot--something not even Zeus can do--and he dies in the attempt. Bellerophon tries to ride Pegasus to Olympus to be with the gods, is thrown from the winged horse's back, and dies alone and miserable. Otus and Ephialtes try to trap and marry Artemis, but she tricks them into shooting each other. The list goes on. When mortals try to act as though they are equal to immortals, it always goes so poorly for those mortals, and Victor--the Modern Prometheus indicated by the subtitle--can be no exception. His crime against nature is that he tries to be like the gods, and what he creates is a monstrous thing that frightens everyone and looks like no one. His creature is a symbol of Victor's horrible pride and mortal failures.