When is the taking of a life beneficial to the lives of others?

While reading Frankenstein after each death, I wonder at what point would it be completely acceptable to kill the monster. When the Monster finds Frankenstein and admits that he killed his brother William, why did Victor not just kill the monster right there? Would this make Victor a bad person for committing the murder of his own creation? Or would he be doing good by committing such an act? For me the question of ‘When is it alright or accepted to kill’ has been a question that has been brought up in our society many times, particularly with abortion. At what point can a fetus be deemed a being? At what point is the destruction of the fetus the actual murder of a child? And if the abortion is not done and the mother does not have the facilities to raise the child and the child is raised in a bad environment and neglected and then in turn becomes deranged or evil or harmful to others, then should the abortion have been done? But also how can you know what will happen because every child in my opinion deserves a chance, but every single circumstance and every single thing that is affecting that child’s upbringing should be taken into consideration. So the overlying question is, when is it acceptable to kill a sentient being and at what point would the death of a being created in some way by someone else be a completely positive and accepted thing? If Frankenstein had destroyed the monster immediately, he would have been ending a life, but it would have been a life that he created and it would have been a life that was capable of evil things. But what if the monster had not been affected in the way he was by others? IF he had not been ugly then he would not have been shunned and neglected by others and would have been treated better and in turn been a benefit to society. Some people say it is impossible to tell what a child will be like when they grow up, but looking at the circumstances that surround them as they come into this world can be used to get an general idea of what path this child’s life might lead. It is such a touchy subject but I am curious to hear other people’s thoughts about at what point does murder become something good or beneficial to society or if it never does?