If existentialism according to Sartre hold everyone responsible. Why Sartre said "men are condemned to be free"?
Sartre (late in life interview with Benny Levy), Alasdair MacIntyre and György Lukacs were all concerned with developing an ethics towards the end of their career. MacIntyre is still alive, I think.
I guess you could say the problem of ethics became acute because of Stalin. [and the later Kchruchev speech to disgrace the Stalin legacy]. But Sartre as far as I remember would not join the French Communist Party. This Party was extremely doctrinaire and would not even approve the 68 revolution because it was not pure enough.
What is important and the book you need to find is an earlier book by a Frenchman, Francis Jeanson, “Sartre and the Problem of Morality” , which had to answer the the question left from Being and Nothingness and Sartre’s speech at Club Maintenant, Existentialism is a Humanism. This goes to the title of your question.
The book by Jeanson Here: https://www.amazon.com/Problem-Morality-Phenomenology-Existential-Philosophy/dp/0253166039this
Where is the right and wrong here? The good and bad? Marx himself thought there was a good and bad. He had that figured out at 16yo.
There is a member here from Russia who is a Sartre expert so he can give a much better answer than I can, if he sees your question.
Francis Jeanson was a real person. However, I have often wondered if Sartre ghost wrote the book. I’m probably wrong.