It would make sense that Abaddon would be the figure of contention, as he is portrayed across many mythologies as a fallen deity or prophet. He is also referenced in many games as a bringer of the end of days. This makes him seem somewhat like Satan, but from what I understand he operates almost solely in the mortal realm. So yes, he is very much like Morgoth from the Silmarillion in many respects. In addition to being a philologist (scholar of linguistics, specifically the manipulation of languages), Tolkien was very spiritual yet antidogmatic. These attributes in tandem allowed him to write of evil and good with a fair bit of objectivity. His writings have had an immense influence on the realm of fantasy, and I would think Guild Wars is no exception. My experience shows me that Tyrian and Canthan prophecies, as far as characterization is concerned, seem to hold that same subtle objectivity. Maybe it's better labeled as acceptance of destiny, but the style remains.
Now that I'm off that tangent, I'll try to shed some light on Abaddon's position from a grammatical standpoint. Abaddon comes from the Hebrew "Avadon," meaning "destruction." He appears in Revelation 9:11 as a demon and "Angel of the Abyss." His name is also used in Satanism, where it also means "destruction." Now, what does this have to do with Guild Wars?
The coming of Abaddon is told in a fashion much similar to Revelation, a dark yet objective style similar to that of Tolkien. The storyline, as far as I've seen, also mirrors much of Revelation in terms of content. ArenaNet, albeit a division of Korean-based NCSoft, is a western company, and it makes sense for them to tell the story in such a way. The fact that Abaddon is an outcast, while not directly stated in Revelation, makes sense based on his title "Angel of the Abyss."
That's just my two cents. There's a good chance we're all mistaken about a great many things, but it's still fun to speculate. If anyone can prove me wrong or make me look like an idiot, please do so.
I would rather be embarassed once than wrong over and over.
Just to reiterate, the correct spelling is "Abaddon."
Sources:
Wikipedia Article
GuildWiki Article
Wikis are unreliable at best, so I also checked a Hebrew dictionary and scanned Revelation.