some of their beliefs aligned pretty closely if you ask me. both very authoritarian to say the least
That was literally the point. They were both radical fascists. One believed in reformation towards an 'Empire of Old' while one rapidly industrialized the economy hoping to create a 'World of the Future'--showing that authority can be progressive or regressive, having no inherit bias.
if you look at Stalin and Riddler you'll realize their political beliefs were actually quite similar, hence the term National Socialists
That's not really the case. The 'Socialism' of national socialist Germany was not what we assosciate with modern progression. The desire to establish a 'Socialist State' in which the government takes control of the economy is not inherently to begin the re-distribution of wealth. There were wealth-elite in Germany but many, many poor.
The 'Socialism' of the Soviet Union also sought to establish a government-controlled market, but for the redistribution (and effective abolishment) of wealth. Socialism can be both liberal or authoritative or both. American Socialism is liberal, while Eastern European Socalism has historically been authoritarian.