First, mythology. The subtitle to the original novel is 'The Modern Prometheus'. In Greek mythology, Prometheus is the Titan empowered by Zeus to create mankind in the image of the gods. The Titan makes Man from clay and water, and teaches him to hunt, read and heal the sick, but Zeus withholds the gift of Fire. Prometheus steals Fire and gives it to Man, and Zeus condemns him to eternal punishment. In a Latin (Roman) version of the myth, Prometheus is punished not by the gods, but by his creation, Man.
The Prometheus myth fascinated writers of Mary Shelley's period, partly because of the advances of science and technology during the 18th C, man-made discoveries which held limitless potential, and potential horror. In the early days of electricity, it was not beyond imagination that this power could be used to create life itself. In the novel, a scientist can and does 'play God', with ultimately distastrous consequences for himself and those he loves. Shelley isn't necessarily 'anti-science', but the novel carries a moral warning against its misuse or under-estimation.
There is a strong allegorical element, too. If Victor plays 'God', then the Monster is his 'Adam' - first created by God 'in his own image'. Once Victor rejects the Monster as hideous, he becomes Man alone in in a cruel world with an uncaring God - not inherently 'evil', but who will learn evil from others. The Monster, as it learns to read, compares itself with Lucifer, the Fallen Angel from Milton's Paradise Lost, a creature who rails at his Creator for giving him a mind and a capacity for feeling. In both aspects, 'evil' is not straightforward or absolute. The act of creation is potentially dangerous, not just for the creator, but for Mankind.
There is great sympathy for the Monster, and with its desire for revenge for its existence. The implication is that, had Victor allowed the Monster into his life, it could have become gentle and civilized. In the end, we cannot really tell Victor and his Monster apart, except that the Monster is taller.