There are different secular value theories. One is social contract theory. That imposes a uniform code of conduct. However, it’s arbitrary. It varies in time and place. Different societies with different social mores, or even the same society with different social mores at different times.
Another is evolutionary ethics. A problem with that is the even if natural selection could confer moral instincts, this wouldn’t make our instinctive predilections objectively right or wrong. Moreover, moral instincts would vary according to the species. Different species with different moral instincts.
Let’s consider both these positions from another vantage point. Alien invasion is a common theme in the SF genre. In one variation, earth has a rare natural resource which the aliens need to survive or flourish. In another variation, aliens use humans as experimental test-subjects.
From a secular standpoint, there’d be nothing wrong with a technologically superior alien civilization exploiting us. The alien race has its own social contract, based on alien cultural values. Likewise, the alien race has its own species-variable moral instincts.
There’d be nothing wrong with aliens using humans for involuntary medical research, just as we use animals. We experiment on humans rather than animals because we value humans more highly than animals. Why experiment on humans if an animal will suffice? Better to sacrifice an animal for our benefit.
Likewise, there’d be nothing wrong with aliens exterminating the human race to monopolize our natural resources. They have a different social contract: a social contract by and for aliens.