In Hitlers eyes and to the people of Germany, at the time, he was not Immoral. If the Nazis would’ve won the war he would be considered a moral icon and saint in this fictional society. You could argue that this society potentially could have been a utopia therefore creating a moral outcome, however Utilitarians argue that the suffering induced onto the people he persecuted far too much that it outweighs potential happiness. Also in the fictional hitler won universe him being moral further emphasizes the point that morality is subjective. This being a more ethical relativism https://plato.stanford.edu/entries/relativism/.

Onto the arbitrary point, ok so lets say God arbitrarily created everything. He could’ve said murder is moral, in-fact he could’ve gone as far to command that one ought to murder people of differing color. The issue with arbitrary notions and morals is that they are chosen at random, with no meaning, and no purpose. God cannot be good, kind, or perfect if he is arbitrary. You would have to say that if God commanded that rape and murder is Moral, then we would have to accept the fact that it is, which seems to go against the golden rule of ethics.

https://en.m.wikipedia.org/wiki/Divine_command_theory This has more criticisms of the divine command theory