Under moral subjectivism, good and bad are entirely subjective commodities. This means that if I think a thing is right, it is as right as is possible for moral right to exist. The principle of subjective morality authorizes an act as “morally good” if the person that performed the act believed it to be the right thing to do; that is the only framework available to moral subjectivism for an evaluation of “moral” and “immoral”. It is strictly a relationship between the actor/believer and the act.
Therefore, as long as Hitler believed his actions right, and those who carried out his orders believed similarly, then to the full extent that the principle of moral subjectivism has to authorize anything as “moral” or “good”, the holocaust was a good and moral event, and moral subjectivists must (rationally speaking) admit this. (I doubt they will, though.)
The way that moral relativists attempt to wiggle out of this is by saying that in their opinion, Hitler was behaving immorally. Unfortunately, they have no rational basis for making this statement. It is a category error, a non-sequitur under moral subjectivism, offered as if there was some means by which to pass judgement on what others consider to be right. Their principle necessarily endorses the actions of the Nazis as morally good as long as they (the Nazis) believed what they were doing was right; what anyone else thought or thinks is entirely irrelevant. The most that the principle of moral subjectivism logically allows subjectivists to say is that gassing the Jews would not be morally good for them personally to do, but that it was morally good for the Nazis to do.
Furthermore, since the principle of moral subjectivism offers no valid reason to intervene in the moral affairs of others (since it is entirely subjective and there is no objective obligation or authority to do so), and since moral relativists must admit that nothing morally wrong was occurring in the first place (in fact, only moral good was likely happening, since the Nazis believed what they were doing was right), they must hold that we should not have interfered with the Nazis.
Thus, moral subjectivism necessary means that the Nazis were doing good and we shouldn’t have stopped them.