Slavery is a socioeconomic system in which slave owners own all the property and have complete control over the lives of their slaves. Western society had to put lots of effort into describing people they enslaved as sub-humans to morally justify slavery.

Socialism is a socioeconomic system in which the government owns all the property, and the government's bureaucratic machine fully controls people's lives.

Fascism is a social system, the rule of elites in which individual rights are entirely subordinated to the common good, and the "common good" is defined by the ruling elites.

National socialism was a fascist ideology of socialists obsessed with the common good of a race. Those who didn't belong to the Aryan race were designated to be enslaved or exterminated.

Communism is an ideology of socialists obsessed with the common good of whoever communists describe as an "oppressed" class or demographic. Those not "oppressed" are "oppressors" who deserve to be enslaved or exterminated.

This means that socialism is slavery to the government, and nazism and communism are just two subsets of fascism. Sympathizing with communists is sympathizing with fascists; supporting socialism is supporting slavery.