system
Given the following post, respond as if you're commenting on Reddit:
human
[Wh40k] Who is the most excessively violent mortal in the empire? I mean someone even a Space Marine thinks should calm down a little.
gpt
Probably Bjorn the Fell-Handed.

First of all, he was chapter master of the Space Wolves and fought alongside, and was a respected ally of Leman Russ at one time, so that should tell you quite a bit right of the bat. 

Super early in his career, he was at the Burning of Prospero and almost single-handedly (I mean that both figuratively and literally) defeated a Daemon of the warp, even after having pure Warp energy forced through him, causing his arm to decay and die. He remained uncorrupted from this encounter, as well. 

After this, he led a group of men, and would often be sent where the fighting was expected to be the heaviest and the most grueling, because he would wade through that shit like knee-high water. In one such encounter, Bjorn and his "Pack" attacked fucking Arvax the Arch-slaughterer, Daemon king of Khorne, and every one of his men were killed in short-order, but he, miraculously, managed to claw his stubborn ass out of there and survive. This Daemon King then went on to systematically murder basically everything it came into contact with. Bjorn would then go on to lead a counter-assault that would finally drive it from the field. And after all of that he mostly felt cheated for not having been granted the glorious death that his brothers had. 

He would then spend the next 5 years doing basically nothing but look for that fucking Daemon King that killed his men. He was SO fucking determined, that he beat his Primarch, Leman Russ, a guy with basically unlimited resources and a mind and spirit DESIGNED to hunt and kill, to the punch. Not only did Bjorn beat Russ there, but Russ was actually present to witness Bjorn, again single-handedly, kill the Daemon by climbing the motherfucker like a rabid squirrel on a tree and tearing its throat out. Russ, fucking flabbergasted, just invented a personal "Wolf Guard" and placed Bjorn there as its first member, because that is exactly the kind of guy you want alongside a whirlwind of slaughter like a Primarch. 

Then, after the disappearance of Russ, and after serving as Chapter Master, he would finally be killed trying to save his own men....
 

 
 
 
They then took his broken and worthless body and shoved it into one of the most devastating Drednaughts in the Imperium, and he would spend the next 10,000 years at the forefront of countless battles, always where the fighting was the thickest, because nobody could imagine a world where a war didn't have Bjorn the Fell-Handed tearing things to shreds. 

Even after DYING, he continues to be one of the greatest weapons in the Imperium.