The sheer wastefulness, plus indiscriminate killing even by the standard of IoM.
The Death Guard later finished the job.
Even DAoT Humanity, by every evidence, was a pretty nasty piece of work.
The answer to the former is because they genuinely believe the Emperor was right and that Angron's assessment of his tyrannical reign was incorrect for reasons, or out of hypocritical disgust where they found his methods icky but not the core ideology which informed the choice of those methods.
That said, it must be made clear that Horus had hands in turning the Censure into a mission of total annihilation. He basically engaged in a smash-and-grab of the Prosperine psyker population.
He didn't have to do much to push Russ into committing to a campaign of total murder; as you observe, Russ was just the man for the job in any event, and this appears to have been a sentiment widely shared.
Valdor was right there telling him that the orders were to bring Magnus back alive to Terra, Prospero's destruction (and apparently a bunch of TS related worlds) is on Russ.
Which Russ at least listened and did try to talk to Magnus.
For any other work than imperialistic genocidal conquest, he was exactly the wrong man for the job.
You know, it's probably why he adamantly wanted humanity to remain human and ruled by humans, not the Primarchs or marines.
Objection! Primarch II and Primarch XI did nothing wrong!
He made it look gratuitous.
But only after ensuring that the leadership believe in his ideals and basically do what he'd want without any prompting and genociding anyone that doesn't fit within his ideal of humanity.
One of them hogged the sharpshooter niche and then died so now the only shooty Primarch is Perturabo.
I like to think their unspeakable crime was nothing more or less than total resistance to the Imperial Truth.
He had a funny way of taking himself out of the process then.
For example, the earliest combat record of Rhino is DAoT Human colonists using them to genocide a primitive alien race.
Like the Bloodtide and Phosphex.
I was also thinking of several worlds the Imperium fought during the GC that are stated to have retained DAoT techs. Many of them were some kind of totalitarian dictatorship with active sadism.
He probably thought he could eventually go back to being a background force like he did for most of history.
Ironically, he was unquestionably a force of good in ancient (and medieval times), when bronze age warlord shit was norm and he was fighting Void Dragon and hunting down Chaos cultists as a knight.
Well according to that one short story anyway, depictions of DAoT power are all over the place.
Also everyone making their own genemods turned the human genome into a house of cards just in time for a storm to kick up some winds.
Humanity is, functionally without exceptions, made up of utter loonies in this setting.
To be fair, given the existence of things like Orcs having weapons like that is rather justified.
Loken agreed to be his sponsor because he felt like someone like him who was willing, to tell the truth no matter what was needed.
I honestly hope that you're not accusing me of performing fascist apologism.
