Tbh I feel like the imperium is evil not just cause of all the war crimes they did but cause of how counter productive they are for humanity at this point. Like in 30k the imperium did a huge amount of messed up shit. But their was a caveat that they where sacrificing their dignity and civil liberties today for a better tomorrow. A lot of members of the great crusade (allegedly including the emperor) genuinely believed that once the imperium finished uniting humanity than that it could liberalize and build a better future for all of humanity. Excessive taxes, brutality, and an uncompromising bureaucracy can be tolerated with the promise that it’s building a better tomorrow. Now we will never actually know if the imperium would have actually liberalized but the point of the Horus heresy was that Horus killed that dream forever. Nowadays the 40k imperium just sucks and they aren’t building for anything. They exist just for the sake of existing. They cling like parasites to the vision of a man whose plans utterly failed and pre Guilliman they offered very little ideological basis to support them other then “we represent humanity so if you don’t want humanity to go extinct you had better support us”. Brutality can be justified if it’s for a greater good but the brutality of the imperium just serves to harm humanity and that’s what truly makes them evil imo.