They attacked Stalin and threatened the existence of Marxism
They were Socialists (National Socialists) but they lost. No one likes a loser.
The Left does not like the Jews. Never has.
The Left lies about everything. Always has.
If you ask a Leftist why the Nazis were bad, they won't give you the honest answers (listed above). Instead they will say "The Holocaust". But I don't think that truly counts for much among the Left. It's just a convenient cover.
So, the Nazis are demonized because they endangered Marxism and Socialism.
If someone comes along and seeks to renew the Holocaust, there is, for a Leftist, not much too complain about.
Bingo! When demmonizing mass-murdering Stalinist international socialists becomes as popular as demonizing mass-murdering Hitlerite national socialists, we'll know that our culture has finally taken a turn toward toward intellectual honesty.
One of the biggest lies in popular culture is the idea that National Socialism is a "right-wing" phenomenon.