The intuition that machines cannot be as ethical as humans is likely incorrect. Ethics is the systematic resolution of conflicts of interest under conditions of shared purpose. Ethics is not irrational. There is no reason to assume that machines cannot be more ethical than us.
You may want to first distinguish ethics from morals: a person that has fewer moral constraints can still be more ethical, and vice versa. Then it becomes apparent that being more ethical may enable to understand the decisions of others as ethical (even if disagreeing with them).