From Hitler's writing, it is clear that he despised socialism/communism because socialist ideologues promoted class antagonism and internationalism, both of which undermined the unity of the German people.

Of course, at a practical political level, leftists were also his rivals for the votes of the dissatisfied in inter-war Germany.

Because Jews are the primary enemy in Nazi ideology, and the idea of communism being a Jewish plot was an existing antisemitic belief well before Hitler was a known entity in Germany's political scene: https://en.wikipedia.org/wiki/Jewish_Bolshevism

Communism was the main competitor with Nazi ideology. They offered competing solutions to the problems of the downtrodden and destitute population. As the main competing vision, it was also the main threat to nazi success.

Hitler was a nationalist, the Communists were internationalists. Communists wanted to improve the lot of people, Hitler wanted to improve the lot of the German people. A random German could become a Communist, but a random Communist would never spontaneously turn into a German.

So the same reason why all kings hate universal philosophies/religions. They bypass border defenses.

How does that fit with the Nazis forming alliances with Italy, Japan, Bulgaria, Croatia, Thailand and even China at one point? Hitler agreeing to a pact with the Grand Mufti of Jerusalem to further German nationalism in the Near East? Then they had Muslim SS officers that had to stop training multiple times a day to pray to Mecca. Because Hitler was so anti-internationalist, the Nazis were eager to host the Olympics and invite people from around the world to show off their regime? What were they doing in Africa, were they defending the interests of the German state in the Sahara?

Don't be blinded by their propaganda, both Stalinism and Hitlerism and any totalitarian ideology is inherently internationalist. Because they tolerate nothing beside and beyond their own ideology, so they end up getting involved with the entire world always. It's about control and these regimes do not stop until their are either defeated of control the entire world.

Dictators hate other dictators. There can only be 1 winner and Stalin was his main competitor.

He probably saw what happened during the Holodomor

He'd either not care, or actively cheer for it. More Lebensraum!!