The German Communist Party (German: Kommunistische Partei Deutschlands or KPD) was banned by Adolf Hitler the day after his appointment as Chancellor on 30 January 1933. It was the Nazi party's first priority to eliminate all political opposition and consolidate their power. However, the primary opposition to Hitler was the KPD, and the Nazis had specific plans to destroy them. On 27 Feberuary 1933 the Reichstag was almost destroyed by fire. Per Wikipedia:

" Marinus van der Lubbe, a young Dutch council communist was blamed for the event. The fire was used as evidence by the Nazis that the Communists were plotting against the German government and the event is seen as pivotal in the establishment of Nazi Germany."

With the Nazi Party firmly in power and the KPD outlawed, German Communism would never be a powerful political force again. Thousands of KPD leaders were persecuted and sent to concentration camps. Many members of the KPD left Germany for the Soviet Union, while others went underground and continued to operate in secret. In the post war years the KPD won several seats in the Bundestag (West German Parliament) but with the establishment of Soviet control of East Germany they quickly went out of favor. In Communist East Germany the KPD was merged with the Social Democrat Party to form the ruling Socialist Unity Party, which governed East Germany (under tight Soviet control) until the fall of the Soviet Union and the unification of Germany.

It isn't really pertinent to your question, but the history of Communism and Socialism in Germany is fascinating. Karl Marx and Frederich Engels wrote in Germany after all, and the Socialists were the largest political force before and during the First World War. Even the Nazi party was ostensibly Socialist before Hitler took control and outed the leadership. Hitler saw all forms of Communism, Socialism and Bolshevism as Jewish plots to take control of Germany and therefore despised it. However, he took many of his early political lessons from the German Socialists. Most notably, the realization that to control a country one needed the unyielding support of the labor force. Communism was Hitler's primary competition in the inter-war years, and continued to be the greatest threat to the Third Reich in the form of the U.S.S.R. which led to Hitler's greatest defeats and losses.

I'm away from home and my library so I don't have the sources or quotes I would like. I hope this broad overview helps answer your question though.

Another aspect to this:

According to Wilhelm Reich, author of "The Mass Psychology of Fascism" both Nazism & Communism had appeal among the masses, both ideologies forms of Fascism (Communism = "Red" Fascism).

Upon the coming into power by the Nazis and their eradication of both Communist & Socialist parties, it would have been a minor shift of belief and loyalty to Nazism, as all of the above demand self-repression & subservience from the masses. He came to these conclusions after asking why the masses went against their own class interests by embracing the Nazis.

Needless to say, Reich equally enraged psychoanalysts, Nazis, and Communists so he wisely left Germany.

Although 20,000 select German communists fled to Russia, there was not exactly an outpouring of German leftists.

Some of the most fanatical German defenders on the eastern front were ex-Communist party members, sometimes in penal battalions, a certain death sentence given the fighting in the East. Still they would fight to the death.

Oh yeah, he also p*ssed off the U.S. Govt too.

As an old adage goes, if you p*ss off so many around you, you probably are onto something significant.

tl;dr German ex-Leftist made the switch and became defenders of German, even though it was fascist.