I haven't beat Darnus quite yet but why I'm leaning towards the Shadow Lord His vengeance isn't only of his own accord. If you played a Galka in XI you can almost sympathize with him. It seems to me that Darnus is a "loose cannon" even to his own people while Shadow Lord's motivations for his rise to power were somewhat predictable. If you played a Galka you might be thinking, "Hmmm. 'bout time."
It caused a great deal of introspection no matter what race you were, and really set the stage for the thematic focus of the Ark Angels in The Rise of the Zilart. All of the different races "stigmas" were already being put into the forefront prior to the first expansion and the Shadow Lord's very existence is how that theme was introduced.
So far I'm leaning Shadow Lord because his motivations were beyond himself, and when you discovered his identity the pieces just seemed to fit. Seeing the post above really makes me want to beat Darnus though. I'd like to see what's considered "rational" in the eyes of this supposed power hungry madman.