"Ethics" is subjective, there is no such thing as an objective ethics (unless you are religious).
This is why thought experiments like the Trolley problems don't have a simple answer.. and they get even more complex with variations of the thought experiment.
I agree ethics can be subjective, so much so that there is more than one model of ethics.
However, please tell me which model of ethics supports eugenics. Is it Utilitarian, Rights, Fairness, Common Good, or Virtue?
Since it's subjective, you can just make up your own model supporting eugenics, and it'll be just as valid as any other model.
but it may not be accepted or viewed as moral by others