>idea
the wehrmacht had over 13 million soldiers. each and every one of them being a war criminal personally involved is well.. "highly unlikely". or to put it on it's head absolutely no allied soldier committed any crime whatsoever - because they're obviously the good guys (inb4 "well they could've deserted, why did they fight under the nazi banner? that makes them evil!"). and that's why that whole good/bad dichotomy falls apart right from the start.
I'm not saying that every Wehrmacht soldier was a war criminal, but some certainly are, especially in the early days when it was the Wehrmacht that was given the task of killing undesirables, but obviously, some Wehrmacht soldiers didn't agree with the Nazi ideology (which led to some of them defecting to Allied hands) and there's also tons of war crimes on the Allied side, particularly with the Soviet Army (there's a reason German women didn't want to end up in the hands of Soviet soldiers, particularly after the brutality the Germans exhibited during Operation Barbarossa). It's just that the idea of the Clean Wehrmacht has been disproven a long time ago, especially by WW2 buffs who studied this stuff.
just like bad people can do good things and vice versa authors and creators can be dumb.
The franchise obviously has many authors, and you eventually get your Karen Travisses or Matt Wards who idolize an in-setting faction to the bone, as Rick Priestley complained.
or let me put it this way: what is a hero? someone fighting for truth and justice? protector of the innocent? defeating evil? no surprise humans identify with the human faction in fiction, written from a human perspective with human values, first and foremost - and that's why it's a brainlet take, on top of missing the point. said innocents that exist within a faction full of assholes will laud a space marine that protects them from xenos. why? because otherwise it would likely mean death or worse. what they believe in while doing it makes zero fucking difference. another is the already mentioned "well if they're part of the imperium they must be assholes because everyone is". and least of all what fucking choice do you have? how could a oldcron (fuck the nulore), tyranid, orc or cultist be heroic? that only leaves tau and eldar and squats, and last time I looked people tended to put them down on the good guy's side.
A hero is someone who fights on the side of GOOD. Not just someone who protects. The Nazis protected their people from Communism, does that make them heroes? Someone who protects can be an evil person fighting another evil for personal power, (for example, Chaos forces protecting their worlds from Tyranids or Necrons) or someone neutral who was just paid to do a job, but a hero is willing to get out there and save your ass even if you didn't ask for it, even if the people he's fighting pose no threat to him whatsoever.
Meanwhile, the original authors of 40K DESPISED the fact that Space Marines were being written in as heroes by the later crop of authors, because they originally created the marines as self-delusional brutes enforcing a regime that is built on bullshit, a regime that, as others have pointed out here, inflicts just as much, if not more harm unto mankind as the enemy does. At least the Tau are willing to give you a comfortable life, the Chaos factions will allow you to let loose and have fun before you inevitably devolve into an animal, and the other aliens are honest about wanting to kill you; the Imperium, for the most part, works people to death or sends them to die as cannon fodder. So while there are genuinely good people in the Imperium, the whole faction for the most part are assholes, and Guilliman knows this.
you can absolutely have good guys fighting bad guys with a clear story arc if done it properly. that's why people call lamenters that fight to the last man to keep a planet from turning into another nurgle petri dish heroes - because they do heroic things. just like they will call any eldar or tau that fights against a corrupt inquisitor or dark eldar the same.
it's a matter of scale, and that's where things start to break - brainlet customers ask for stories involving important characters, completely oblivious that involving said characters will have wide reaching consequences in the lore because they're important characters. like, what would happen if gullyman gets #metoo'd in a book?
But that's not Warhammer 40K. In the end, ALL the factions are assholes. In fact, when they tried to introduce the Tau as a heroic faction that even takes in human deserters and treats them well, the fanbase ERUPTED IN A FURY and said that the Tau don't fit in 40K because they're too good. So while individual humans in the Imperium can be good or heroic, the faction they work for is not, and even some of those "heroes" enforce that brutal, corrupt regime unto their own people, casting doubts on whether or not they were heroic, or just following orders. A real hero does what's right, regardless of orders-being a good hero means doing what you know is right, regardless of what they're told to do.
40k worked as long as GW kept conflicts and stories local, after all they're just some made up backdrop to push plastic into each other. trying to turn it into another MCU and have it connect with everything is the moment where it falls apart because neither the lore nor the setting can work this way without making no sense and/or just sounding stupid. which is the whole reason we have this discussion in the first place.
That's exactly why the lore nuts are pissing their pants; they want to keep stories local and grimdark, so that status quo would remain as a backdrop for people to have a setting where everyone fights everyone and your personal fanfic story of the humans fighting both the Orks and the Eldar on the same day would fit in the greater narrative. When things start changing, the Imperium starts getting stronger, they start innovating tech-wise (ex: Primaris Marines) and they are potentially building up an alliance with the Eldar, the lore fans begin to disagree, since the whole grimdark story where everyone hates everyone is slowly being swept away.