The Latin word sociare gave birth to two different words, socialist, and Nationalsozialismus(Nazism). While they had the same root, they were two very different ideas. The root word means a team, or a group working together. Socialism was an economic system where people work together, while Nationalsozialismus is a governmental system where a nation (ethnically similar people) work together. In German, if Nazism was a form of socialism, it would be two words National Sozialismus.
The Socialists were the only people who voted against the Nazis in the Reichstag. The other parties fell into line, or like the Communists had already been outlawed. The Socialists were immediately outlawed by Hitler, using the Enabling Act. Their leaders were killed, or at least tortured.
Bizarrely enough, the survivors were willing to work with the Christian Democrats after the war, even though the Christian Democrats had worked with the Nazis. Even today, there is broad coalitions in Germany to keep the right wing radicals out of power.