Why didn't Frankenstein just kill his creature after it killed his little brother William and framed Justine for the murder? For that matter, why didn't he just kill it after he first brought it to life and realized that he had made a horrible mistake? Well, for one thing, Mary Shelley wouldn't have had much of a story there. It would have been pretty short. Instead, Frankenstein's feelings toward the creature became even more hateful and then, to save the rest of his family, he felt he had to submit to the creature's request to create for him a partner.
Frankenstein did suffer some mental anguish in his decision to grant the creature's request, but pretty quickly decided to go ahead and take the risk that he might be creating another raging superhuman killer instead of a loving companion that would subdue the creature's violent quest for revenge. When a parent gives birth to an unwanted child do they have another child to keep the first one occupied and distracted so they can have their own carefree life back like Frankenstein wanted? Pretty silly if they think that's going to work!
In fact, some parents, for whatever desperate reasons try to dispose of an unwanted child soon after the birth. Take, for example, the scandalous case of Amy Grossberg and Brian Peterson in the 90s who concealed Amy's pregnancy, then delivered the baby in a hotel room and tossed its dead body in a dumpster and tried to go on with their lives as though nothing had happened. Comedian Bill Cosby tells a humorous story of his father warning him that he brought him into the world and he can take him out too and just make another one who looks just like him.
Human beings have the power to create life and to destroy it as well. There are legal consequences and moral responsibilities set by societies and governments regarding both. But Frankenstein created his creature in secret, so why doesn't he just take the creature out in secret? It seems as though he has the power to create life but doesn't have the power to kill what he has created.