A blog post of mine last month, about the 70th anniversary of the world's first atomic bombing, at Hiroshima, is the basis of this.
At least one liberal to left-liberal friend asked if I thought the bombing was "justified."
My response? I said that I generally tried to avoid that word.
Why? I pointed them to my review of Walter Kaufmann's "Without Guilt and Justice." The title it self should offer some hint as to my thought. The fact that Kaufmann is the world's best mid-20th century expositor of Friedrich Nietzsche should offer yet more. But, click the review link for details.
Of course, existentialist morals can differ from one another, Nietzsche's own are surely different from Kaufmann's, and this is his book, not Nietzsche's. Both are surely different again from Sartre's or Camus'.
The big point is that he disagrees strongly with John Rawls on ethics, and his critiques surely apply in a general sense to other liberal humanistic schools of ethics.
Perhaps a better way of looking at this is that Kaufmann believes in his own version of humanistic ethics. It is an empirical one, and one that fully accepts the Problem of Induction in trying to formulate moral laws.
And speaking of induction and empiricism, perhaps this is a good time to reference David Hume's "An Enquiry Concerning the Principles of Morals," which in turn, may have influenced Nietzsche's "On the Genealogy of Morality." (Yes, he reacts against some British philosophy, but that appears to be against utilitarianism.) Certainly, it's arguable that Kaufmann sees Nietzsche through a Humean set of glasses, to the degree either one is influencing him.
I take Hume's focus on "the passions" and morals as descriptive, rather than prescriptive, and certainly not an overarching theory. Of course, I think Hume rejected such theories in general, and that itself is arguably a theory, or an anti-theory.