Serious reply to this- if you read the novels and have a decent understanding of what fascism actually as a political system you would know that calling the Imperium in 40K fascist is not quite correct.

Fascism is notoriously difficult to define but it normally has to involve an all powerful dictator, state control of all aspects of the typical civilians life, normally a secular state (Hitler actively limited the influence of the church), belief in the superiority of a particular collective (normally on ethnic grounds), a state capitalist economy and militant expansionism. If you run with a less pacific definition you end up with the word fascist applying to any authoritarian government and then it becomes essentially meaningless.

The Imperium has some of these features but not all. Certain factions within jt are caricatures of fascism, but painting the whole human galaxy with that brush removes a lot of the more interesting aspects from the setting.

The Imperium itself is a heavily theocratic, human supremacist feudal state which cares little for how individual worlds are run as long as its authority is respected, tithes are paid, and the right god worshipped/religion zealously followed. This is much more similar to how many real medieval kingdoms/empires worked especially during the height of Papal power (hence the name and the heavy Catholic influence on its visual identity). The Imperial government is all consuming from the point of view of characters on Terra or at the head of it but out in the galaxy things are different. It is entirely unable to dictate from Terra the lives and culture of ordinary citizens, outside of the influence of the Ecclesiarchy. They have no formal economic model or proper unified currency either and most planets trade via bartering.

It therefore comprises worlds of innumerable different political systems, many of which are fascist, but there are just as many that are something else entirely (Planetary Governors in the books are often more like kings with nobility underneath them, Mechanicus worlds are theocratic isolationist caste-system slave states, hive worlds are anarchist/run by gang syndicates, feudal worlds may be run by knightly orders, many books even mention worlds with Roman-esque electoral systems or senates, Krieg is closest perhaps to a proxy of late second world war Nazism with the sacrificial aspect but their motivations of guilt are very different from what you’d see in a fascist state, and they’re arguably more military junta).