It is disingenuous, I think, to claim that Hitler's national socialism and Stalin's socialism belong on the same 'leftward' side of the political spectrum. 'Left' and 'Right' appellations are a shorthand of convenience, for one thing, and misleading in this context. The nazis, although sharing the usual totalitarian disregard for human life, certainly derived nothing from either Marxist or Leninist economic models. If one is talking about American politics, then 'left' and 'right' are commonly understood to mean those domestic political parties descended from American historical tradition--political groupings literally reading from the same page; e.g., the US Constitution. Followers of Hitler, Stalin, Mussolini, and Osama bin Laden (and so on) fall into another category entirely. The comparisons and associations many folks are making here are simply invalid.
A vocal section of world socialism finds it embarrassing to admit that Hitler was indeed one of their brethren.
I understand why some have a problem. Among those are some that do not realize they are being used by their own.