Final answer:
Victor destroys the monster's companion primarily to prevent the creation of more monsters, acting out of fear for humanity's safety.
Step-by-step explanation:
Victor destroys the monster's mate before his eyes to prevent the creation of more monsters, fearing the potential consequences of their existence and propagation. This act, while it also carries elements of demonstrating power and seeking revenge, is primarily rooted in Victor's apprehension of unleashing a new species upon the world that could lead to further devastation and terror. His actions are a mix of self-preservation and responsibility toward humanity, as he believes the existence of another creature similar to the monster could have disastrous outcomes.