@Goldenzeal Nodhoggr means "Malice Striker", as in one who strikes malice. The Nid part of his name even denotes the state of being sinful and dishonorable by Norse standards. Eating the sinful dead has always been his main thing, and not being sinful has always been a main thing in Norse myth, even if their standards for virtue were a bit skewed compared to their contemporaries. He's always been the scariest thing in the entire realm of death.
Fafnir was just a greedy dude who got cursed (into being a goddamn dragon), and a siegwurm is just not even a thing at all. The author probably gave the name in reference to Sigurd, a dragon slayer, because of the medal the creation one turned into, but that's just tentative as fuck. The guy just sucks at character concepts, honestly. I mean, the sequel novel to this has basically a fully feminized version of Rimuru straight up imported into it.