What if murder became advantageous?

If you think murder is advantageous, then you would commit murder, which is exactly what some people do...that's why it is subjective. If morals were objective, as you keep suggesting, then everyone would think the same things, but obviously they don't.

Secondly, what Hitler did was called "eugenics", which is quite literally described as: "the study of how to arrange reproduction within a human population to increase the occurrence of heritable characteristics regarded as desirable. Developed largely by Sir Francis Galton as a method of improving the human race, euge…  Read morenics was increasingly discredited as unscientific and racially biased during the 20th century, especially after the adoption of its doctrines by the Nazis in order to justify their treatment of Jews, disabled people, and other minority groups."