One philosopher who had a difficult time accommodating ethical considerations in his overall philosophical view was the late French "existentialist" Jean-Paul Sartre. The thrust of the book by Simone de Beauvoir reprinted in the text is to claim that an ethical theory is compatible with Sartre's view of human existence.
Under the slogan "existence precedes essence," Sartre proclaimed the radical freedom of the human being. Although we are mired in definite objective circumstances (what de Beauvoir refers to as 'facticity'), we have many choices open to us. Indeed human consciousness itself is nothing more than the kind of thing that can take into account alternative possibilities. Everything else in our world is fixed.
Although the human being must be free and is bound to some extent by circumstances, all else up to the individual. One can face up to one's freedom or try to escape it. It is easy to pretend that everything is out of control, to hide under the banner of determinism. Freedom is hard to face.
The reason freedom poses such difficulties for Sartre is that there is nowhere to look for guidance about how to use it. He denied that there are any moral rules to which we can turn ("laws of freedom," as Kant paradoxically put it), particularly since he denied the existence of God. So it looks as if any value placed on anything is purely arbitrary, and that Sartre is forced to subjective relativism. (One of my Ph.D. students argued this point forcefully in his recent dissertation.)
Simone de Beauvoir's work was an attempt to save Sartre's philosophy from this conclusion. She granted the apparent absurdity or meaninglessness of existence while holding out for a standard of ethical worth. This she found in the attitude toward freedom itself. One faces one's freedom when one adopts a life project, when one undertakes to define what one is to be. Given that this is the key action, de Beauvoir endorsed what Aristotle would call the moral virtues of courage, patience and fidelity--precisely the virtues needed to face one's freedom resolutely. Of course, she put them to use differently from Aristotle; these are not excellences in the control of the passions by reason. But I think their status as excellences cannot be doubted.
There is also a Kantian side to this view. Freedom is to be respected, so that one should not aid in others' flight from it, nor should they deprive others of their options. However, this principle is not grounded in pure reason, but only in a recognition of a common human condition.
Existentialism by its very nature is a difficult doctrine to accept. Philosophers have expressed in many vivid way the terror of a completely open future with no guiding light. Thus de Beauvoir characterized existentialism as "austere, sad, but not evasive." It is best accepted by those who have already met many challenges of life head on, who have confidence in their life-projects. For these people, the injunction "Do what you must, come what may!" can be answered without hesitation.