I mean, the Emperor was still a fascist shithead, so he's still awful, regardless.
The Imperium is not fascist.
The Emperor has morals he does not cross (did not cross during the Crusade, though would have if he had to).
Never afraid of extreme measures, Angron had let slip his World Eaters in the most vicious way imaginable. Remus had once heard his primarch say that Angron's Legion could succeed where all others would fail because the Red Angel was willing to go further than any other Legion, to countenance behavior that any civilised code of war would deem abhorrent. Seeing what had been done to Prandium, Remus understood completely. This was no honourable war, this was butchery and destruction embodied. The primarchs' great work could surely never have contemplated war with so terrible a face. -Page 32, Age of Darkness
Most Legions frowned upon Angron and Kurze (Angron for murdering planets instead of conquering them as most Legions). Kurze's officers chastised him for murdering children and broadcasting their screams on the vox net.
The Death Guard used rad weapons, they used all manner of chemical weapons we today irl find abhorrent but they were most often against xenos and so they got a pass. when they did come across a human civilisation they demanded compliance and then irradiated anywhere that held resistance. They wouldn't just massacre an innocent city, they would target non compliant enemies first, they would stop when compliance was achieved.
The night lords share this "would stop when the job was done.....eventually" mentality. It's already known that they stepped over the line and mentioned in dozens of sources that they committed atrocities that ended with them being censored. The difference is theirs was a gradual slide to this barbarism we see during the heresy. They would also in some cases leave the population alive (terrified/shell shocked) when they considered "order/compliance" achieved and almost all of their acts were aimed at non compliant worlds.
The Iron Warriors were almost exclusively siege troopers. They didn't go lay siege to friendly places. They were a meat grinder, they camped out in front of an enemy and kept shooting until one side broke. Their human allies suffered under their direction, pointless deaths occurred, but ultimately they were against non compliant forces that no one else could break, non compliant forces that had already made their choice to fight (and die) and so it was noted that they were a needlessly brutal legion, but it was ignored in favour of results and that their actions were almost always sieges against entrenched enemies. They would go for an immediate strategic attack and stop immediately when compliance was achieved. The brutality exhibited by them was how they went about laying siege to a place (sacrificing allies, wasting lives) but they were disciplined and effective.
Angron and the World Eaters however did not stop. If a planet was not compliant then the planet would die. They wouldn't arrest the citizens or terrify/beat them into submission, they would straight up butcher them. This wasn't the typical "march in with the lads and take out strategic targets first" kind of war that most other legions practiced (though most of them went about it in different ways) they would (and have) rampaged as a bloodthirsty horde across the planet butchering anything that stood in their path, regardless of what it was. There was just a straight up "murder everyone" mentality, and it was carried out, but not cleanly. They used axes, they used melee weapons, they would cut and beat and pulverise their enemies to red smears before screaming and raging to the next unfortunate. There wasn't the "the enemy has submitted let's stop now" that other legions exhibited, they just straight up kept going. In some cases straight into their allies. They killed, they were undisciplined and they were a liability. They killed civilians, they killed men and women and children that had nothing to do with the war, and they did it by overwhelming brutality. The lack of discipline was probably most what disgusted other legions. It's unthinkable for many legionaries to just charge into a fight in a ragged horde screaming and raging incoherently as they butchered their way through whatever was in their path without a plan. There was no tactics, just a hate fueled ragefest of brutal murder played out in whatever battlefield they could create in their bloody assaults.
Compare the Luna Wolves in Horus Rising. (the book where Horus said the Emperor was born in "Anatoly" around page 354) On 63-19, Horus exhausts every diplomatic option - even after the death of a beloved son - before engaging in an operation to utterly destroy the "Emperor's" army whilst minimising collateral damage and without wrecking the planetary infrastructure. Horus goes on to lament that he could and should have taken the world peacefully.
They are a Legion famous for their aggression.
