Through the story, we can argue that there are three monsters: Victor Frankenstein, its Creature and society. However, some of them are monsters off bad decisions, others due to their surroundings.
Victor Frankenstein can be considered a monster due to the fact that he abandonned his monster, he also led to a death because he did not mention that the death of his little brother was due to his Creature. For Victor, therefore, we can say that he is a monster due to the decisions he took in the story.
For the Creature, of course he is a monster because of the killings he did, but it can be argued that the real fault is his surroundings. If society in general and his creator Victor did not reject him so much, he would have probably not kill anyone. Therefore, we can say that the Creature is a monster for both his actions but also that his surroundings led him into this monster.
Finally, for society, it is again both a mix of bad decisions and its surroundings. They should never have rejected the Creature so bad everywhere he went. The Creature had nothing but good intentions and to join a society where he could live. However, we can say that those bad actions can be explained by the surroundings, as people made standards of what human beings should look like, therefore, the Creature, not fitting the standards, is a excuse of their bad actions towards him.