I'm not sure that's correct, there were very few leftists who actually joined with the Nazis, the vast majority of communists and socialists remained opposed to fascism throughout this period. Most of the Nazi's base of support was from middle class conservatives and non-socialist working class people, along with many "lumpenproletariats" who were drawn in by the thuggish behaviour of the Brownshirts. As far as the anti-capitalist rhetoric of some of the Nazis went, it was almost always just another form of antisemitism where they'd rail against "international finance capital" but really meant "the jews that control everything". There were some on "the left" of the Nazi Party that were more vocally critical of capitalism, such as Gregor Strasser, however they were kicked out of the party in the early 30s and then many were killed or imprisoned in the Night of the Long Knives. Strasser's brother, Otto, was also a Nazi who had similar views to Gregor but left the party earlier and went into exile before he could be killed and continued to publish work critical of Hitler. I think Otto can be seen as being further "left" then most Nazis, however whatever socialistic ideas he espoused were primarily motivated by antisemitism and nationalism rather than any concern for the proletariat because he was still a Nazi cunt. As for what some of the rank-and-file Brownshirts did after the purges I haven't read much about, I highly doubt there would have been many jumping over to the communist side, but I do like to think they were probably conscripted into the Wehrmacht, sent to the eastern front and then were immediately domed by a Soviet sniper.