A couple points
First of all, the very word "Nazi" refers to the "National Socialist Worker's Party". Hitler adapted elements of Fascism and Communism in his system, so it's fundamentally incorrect to say that Nazism and socialism are polar opposites.
Second, the political spectrum isn't really linear but more a circle. When you get right down to it, there's very little real difference between extreme left (e.g. communists and socialists) and the extreme right (Nazis and Fascists). Both extremes have bent themselves so far from the norm that they've met each other.