You sometimes hear it said from ethicists of a certain Kantian stripe that anyone who is rational has to acknowledge some basic truths of ethics, and, contrariwise, that one test of an ethical maximum is that it be universalizable: i.e. that it be binding on all rational agents.
Supposing though that there are two sub-species of Homo sapiens: regular Homo sapiens, and Homo sapiens sociopathis. The two subspecies can interbreed but the latter one, act with a depraved indifference to the feelings of other people.
Do we know that that’s not the case? No. It seems like it might well be from a reading of true crime stories, or even Aristotle’s Nicomachean Ethics, where he discusses the depraved female who eats her own young.
It follows from the conceptual possibility of a sub-species of sociopaths that it’s not a requirement on an ethical maxim that it be universalizable. The golden rule, or the Kantian categorical imperative will not be recognized by sociopaths.
Of course we could water down the requirements of universalizability and say an ethical maxim only needs to be universalizable across people who are not defficient in their ability to be ethical. We could do the same thign wtih rationality and deny that sociopaths are rational. But that makes the connection between ethics and universalizability (or rationality) trivially true, and therefore uninteresting.
Maybe this view of sociopathy is wrong, and sociopaths are not a separate species, and are susceptible to therapy and education. But if it’s wrong that is just our good luck.