"If you think the United States could never elect an Adolf Hitler to power, note that David Duke would have become governor of Louisiana if it had just been up to the white voters in that state." Robert Altemeyer
There are stark differences but they are often disguised by reality. Nazism is nationalism personified, Communism is community personified. Neither work very well. In political ideology terms, Nazism is of the right, communism of the left. One glorifies nation over the individual, the other glorifies communal life. Nazism also added racism to its makeup, but that often happens as nationalism is often xenophobic. Hitler's Germany is the extreme Nazi example, hippies or possibly tribal people are the only examples of Communism as it requires a level of cooperation unknown to humans. Nazism tends towards a religious justification, while communism being human centric is more secular.
"What we have to fight for is the necessary security for the existence and increase of our race and people, the subsistence of its children and the maintenance of our racial stock unmixed, the freedom and independence of the Fatherland; so that our people may be enabled to fulfill the mission assigned to it by the Creator." Adolf Hitler, Mein Kampf, Vol. 1 Chapter 8
For the Reader:
"The Meaning of Hitler" by Sebastian Haffner and "Defying Hitler"
Amazon.com The Meaning of Hitler 9780674557758 Sebastian Haffner Ewald Osers Books
The Rise and Fall of Communism - Archie Brown
Rise and Fall of Communism The Archie Brown 9780061138799 Amazon.com Books
If you want to undertand what we define as evil:
Read Stanley Milgram or check out this video:
Philip Zimbardo The psychology of evil Talk Video TED.com
'Becoming Evil: How Ordinary People Commit Genocide and Mass Killing' by James Waller
Amazon.com Becoming Evil How Ordinary People Commit Genocide and Mass Killing 9780195189490 James Waller Books
"We first kill people with our minds, before we kill them with weapons. Whatever the conflict, the enemy is always the destroyer. We're on God's side; they're barbaric. We're good, they're evil. War gives us a feeling of moral clarity that we lack at other times." Sam Keen