Nobody can really because it's complicated. Or, at least, nobody can agree, which is why we have the terms. However, I think the terms have some validity, because the broader concept does.

I mean, is Hitler a murderer? Is your run of the mill burglary gone wrong worse than the Holocaust? Obviously not. So there has to be some kind of understanding of organized death.

I'm not sure "at least it wasn't the Holocaust" is, in practice, quite the defense legal argument it's being made out to be here.