If one were to justify Diamelen's moral degradation, I think that part of it might lie with how she left with Arsat while she was committed to another man.  If this were to be continued, it would argue that she was of low moral character to do so, in terms of breaking her commitment.  Yet, I think that this is a bit harsh in that she was a concubine of this man and that she was not in love with him as much as she was with Arsat.  Another potential level of moral or ethical degradation in Diamelen might exist in how she and Arsat abandoned his brother, fighting on their behalf.  There might be  moral degradation claim made in how there is a certain crulety in abandoning someone in need, the calls of the brother to Arsat resonating while he is being killed.  However, here too, I think that Conrad's main point is not necessarily her moral degradation as much as to explore the fundamental weakness that exists within human beings.  There is a sensibility present in which human beings' weakness in times of struggle is something Conrad explores on a larger level, something that is not constrained to Diamelen, but to all human beings.  It is difficult to push the argument of or the justification of Diamelen's moral stance, as she does die in the end.  It can be argued that whatever her moral defects, she does pay with her life.  In this, I think that Conrad's point is not to seek blame or seek some type of judgment where there is a moralist approach to one's character.  Rather, I believe that Conrad's larger point is to assess how human beings act and what they do within this that helps to define much of their being and how, potentially, our own moral degradation should be studied in light of their own narratives.