Warhammer 40k is riddled with complex and nuanced characters, hero, villain, whatever Ciaphas Cain counts as, are all built around their flaws. Patience versus anger, kindness and envy, humility and pride, temperance and gluttony ext. A running theme is that most characters are just barely able to commit themselves to righteousness or wickedness but this distinction was made by a hairs breadth. Had things gone just slightly differently faction lines would have been redrawn dramatically. The greatest tragedy in the Warhammer universe is that do overs are less common than convincing false messiahs and very nearly the entire pantheon of godlike characters regardless of faction, pine for what could have been.
Hither comes Erebus! Not a spiral, not a checkmark! His character arc is as flat as a washboard! Hes just good old fashioned "kick the baby" evil! That he throws the first pebble that causes the cascade failure of *checks notes* all of reality lol, is pretty funny. That he regrets nothing afterwards is downright hilarious.
In real life it only takes one sociopath whos fetish is to attack the trust based bonds that allow society to function to ruin lives, and while its hackneyed and cliche to have a number of such villains in books, its not really so tawdry or unreasonable to have one or two.
As far as short stories go, this is pretty good! Its also an excellent primer to get a rough summary of the overall 40K premise. That said, Erebus isn't a good gateway character to bring in new readers. His “””arc””” being a list of atrocities he narrowly avoided comeuppance for and his lack of gradation as a character are probably what a lot of non Warhammer fans suspect 40K is in a general sense and if they got the impression that every character was written like Erebus is here they'd probably be run off.