I've never been too akin to those "demon killer", "dark king", "shadow lord" and similar epithets that fall on common and a bit edgy terms that, to be fair, sound childish to me (too much "pssh nothing personnel kid" vibe). From a storytelling perspective, the idea that under the most famous rookie of the era there's a swordsman that apparently hunts his fellow pirates down to the point of being recognized by that must be quite disturbing.
It's a similar vibe to epithets like "doctor death", with that powerful oxymoronic touch that adds a layer of uneasy paradox which I personally find more attractive that something more simplistic like "demon hunter", whose supposed menace comes from demons being something hellish, I guess...?