Even in Anfang, where killing one's opponents was a common practice within the Rankers' society, Eren had set himself apart from the rest with a peculiar way he practiced the undisputed law of "might maketh right."
The yanking off of organs from opponents' bodies while they were alive, chopping off limbs and necks from joints while avoiding the bone hindrance, like that of a veteran butcher with a keen perception of animal anatomy, the brutal execution of weapon attacks even after the critical hit had been made, and the finesse with which he carried out those actions made Eren be seen as a monster in others' eyes.
This was the reason Eren Elijah Idril was the only one titled the Osan Woods' Butcher, even though brutality and the way of killing were prevalent around him.
It was the way he killed and treated his opponents that made him come across as a vile man even to those who were considered brutal characters having seen their fair share of violence.