system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Why were other Legions horrified at Angron being so brutal?

 **"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 behaviour 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 primarch’s great work could surely never have contemplated war with so terrible a face."** Pg.32 Age of Darkness   


This quote shows other Legions were horrified at how brutal Angron was, but why? They can be ruthless as well, just not as much as Angron and Kurze.
gpt
Kurze didn't have huge body counts on average. He'd skin a planet, then the next dozen or so would surrender with token resistance. Angron, meanwhile, did this on every single planet he attacked.

Perturabo and his legion would grind a world's defenders to dust, and to be fair he *did* have something of a reputation for being a meat grinder, but people just plain paid less attention to him than Angron because, you know, Perturabo.

And to be clear, people, including other primarchs, were horrified of the above two. The experience was probably just fresh in his mind and he downplayed the other two for that reason.