Marxism is a much broader political ideology and intellectual tradition that spans from Communists that want to overthrow the capitalist system in revolution to reformist who seek to use democratic means to push their countries closer to a socialist state. Not every Marxist is a Stalinist and in fact some of the harshest critics of the USSR were socialists.

On the other hand many of the terrible things we associate with the Nazis are central to Nazism. There is no Nazism without extreme racism, genocide and eugenics. There is no "nice" version of Nazism because racism is the foundation stone of the ideology.

Also, comparing the death toll under Communist states and Nazi Germany tends to be quite misleading i think. Not because of magnitude (although many estimates that get thrown around in these debates et for communist death tolls are usually either the highest possible estimate or just totally exaggerated) but because nature of the deaths themselves.