So loyal that he bargained with warp entities before the heresy (thats how he lost his eye)?
So loyal he went directly against the edicts of Nikea, the Emperor literally spelt it out for him, had a massive meeting with all the top tier of the imperium, his brothers warned him, his adopted farther warned him, not very loyal it seems.
The entire reason he used his powers (if you read the novel a thousand you would know) was because he wanted to prove the Emperor wrong, he wanted to use the fact he would "save" imperium using his psychic powers to overturn the edict of Nikea.
after he had destroyed the webway, imprisoned the emperor on his throne, almost destroyed Terra, threatened the Emperors well-being, obliterated the imperium's future, i think he deserved the ass raping Russ gave him.
there is also another VERY big reason Russ went all out, Russ really didnt want to do it, but its a massive spoiler, it shows why the Emperor was so harsh on magnus and why Russ came like a ton of bricks to Prospero