The Monster is Frankenstein’s. The first reason he feels he has the right to kill him is that it is his invention. The second reason is that the Monster caused a lot of damage to his family and himself.
Victor Frankenstein believes he has the inherent right to kill the Monster since it is his own creation and birth. He flees from his creation when he first sees it, disgusted by its appearance.
Victor’s primary goal is to avoid facing whatever he has done when they next meet. When the Monster causes more devastation, later on, Frankenstein feels responsible for its actions. He believes that if he killed the Monster, humanity would be spared from what he has caused it to suffer.
Furthermore, the character’s motivation is not only to serve the greater good. Frankenstein resents his own creation for personal reasons. The Monster initially desired vengeance and killed Frankenstein’s family. Frankenstein is driven by similar vengeful motives at the end of the tale when he goes on a murderous rampage across Europe to destroy his own creature.
Because the monster framed Justine for William’s death, Frankenstein feels he has the right to take his life. Frankenstein understands that he has created a murderer who murdered Williams, Victor’s tiny brother.
He’s lonely and unloved, so he can’t know love. In Mary Shelley’s Frankenstein, the monster is a character who first appeared. It’s a creature made up of various body parts in which life is supplied by Victor Frankenstein (its creator) during an experiment.