Actually the Holocaust is a great argument for moral relativism - it shows what happens when morals are imposed from outside instead of by an inclusive agreement among all members of the society.
But there isn't ever likely to be a time when
all members of society agree. It is, rather, a consensus that is developed. The consensus must be broad enough that the number who disagree are small and not disruptive.
I don't think that you can excuse the holocaust by suggesting that the circumstances were imposed. However, the recognized need to keep it secret does suggest that it was not part of the consensus view even if there was a significant background of anti semitism.
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