The chief implication of existentialism with respect to human conduct is this: What you choose to do, how you choose to spend your life, is not as important as the way you choose it. Whatever the choice, it is at least valuable insofar as it is made in good faith. This means it is made in full recognition of the freedom and responsibility that attach to all human choice.
But necessarily, to recognize our freedom to determine for ourselves what we shall be places no constraints on possible choices. This means that any choice might be made in good faith. To choose to be a vicious criminal could be as much an expression of good faith as choosing to devote your life to those who suffer. The question then arises as to whether the fact that a vicious life is chosen in good faith makes that life any better.
It is difficult to know how this disagreement might be resolved. One line of thought we might adopt on behalf of the existentialist says that the life of the sincere Nazi is objectively bad but subjectively good. If this means that, though her life was bad, it embodied those things that were values for her, we can hardly deny it. She did indeed choose those values; that is what is mean by calling her “sincere.” But this does not advance matters. We know what she freely chose. We want to know whether the fact that she choose freely made it any better or not.