The nature of evil that is expressed through the monster's behavior, systematically killing those people that Victor loves suggests that he has no soul. To be an individual without a soul, in my view, amounts to having no compassion for the pain and suffering of others. To feel that to punish someone else is a victory for your own cause, your own pain, your own deficit.
The monster is not just killing, he is killing with the intention to punish Victor, an intentional, premeditated act, which is evil and soulless behavior.
Even though the monster has been created from the parts of human beings, he is not fully human. I would argue that as an abused and neglected person, the monster knows what he is missing, by not having love in his life or else why would he want Victor to make him a female companion.
His deprivation, his empty lonely life leads him to behave monstrously, like some human beings who feel that the world has cheated them, therefore they must lash out and kill or destroy...
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