A rational person can see that major features of a moral system can be defined by objective reasoning. If I prefer to live than to die, killing is wrong. If I prefer to have my needs met rather than neglecting, helping others to meet their needs is correct. If I prefer health… I prefer to be treated with respect… and so on.
Asserting that there are aliens who prefer to die, kill, feel pain, die of starvation, be sick, be treated without respect etc. does not seem realistic as it is not logically possible. How long would those aliens survive? We can only surmise aliens who ignore these facts, which is perfectly understandable, because ignorance is a common state. Someone who pretends that hurting other is moral because “I’m better” is not being objective, which is why living beings clearly spend so much time rationalising.
In this sense, Buddhism’s ethics have some striking parallels with those of Classical Greek philosophers, esp. Socrates.