Your question is a good one that depends on a person's perspective on what would constitute an act so immoral that a person would be identified as a monster.
During the era in which Victor lives, people were not inundated with science in the same way as people are today. Experiments on the human body and carving up a human would have been considered sacrilegious and would have society viewing him as a monster.
Victor progresses to gathering body parts and sewing them together to create a human being. He is aware that if he is successful, that the human will be disfigured and would not be accepted by society. However, his ambition overrides his conscious awareness of the monstrosity of his experiment. His actions are monstrous.
The creature knows only anguish and pain, but is compelled by his connection with Victor. Victor has created a man but is unhappy with the man that he created. The creature is not accepted by Victor as anything other than a creature and it suffers emotional distress at its own existence. Victor's rejection of the creature makes Victor a true monster, because he brought the pathetic creature into the world for his ambition, without regard to the consequences to the creature and he eventually abandons the creature. I believe at this point, one sees the monster that Victor has become.