Besides, just because Gundam AGE has gone ahead and killed a few likeable characters here and there automatically makes it a "dark" story?
Not really, what makes Age so dark is that it manages to draw out war for an extremely long period of time if you think about it. As much as people will say how dark something like Zeta or Wing or even 00, Age has something to it's darkness others simply didn't: Time. Lets look at how long oppressive/antagonistic forces in Gundam have been around to do horrible things:
-Zeon: 1 year plus a few months depending on whether you place Operation Stardust; of course due to how much effort was put in, their remnants would last for a very long time.
-Titans: 4 years if that, the most notorious of the bunch but their first year was more for establishing themselves before getting their rep.
-Neo Zeon: 8 years from the time of ZZ to Unicorn, some of the years between ZZ and CCA are debatable since a "Second NZ War" would imply they stopped for a while.
-Crossbone Vanguard: I'm not 100% sure on F91's time scale so I'd say...... A month?
-Zanscare Empire: Not sure on the exact time scale, although unlike the Titans which were simply brute force they also had a nurturing side to them (at least for women), making them somewhat of a collective universal entity; the use of Angel Halo highlighted this.
-Devil Gundam: Months?
-Earth Sphere Alliance: 15 years (16 depending on how far you take EW) if I remember correctly although not a brutal as much as they were just constant enforcement; it slowly fell apart under it's own divisions and whatnot, the political structure was poor once the gundams started coming about.
-United Nations Earth/Space Revolutionary Army: They managed to survive an apocalypse after 15, but although we haven't seen much of the latter the former could barely maintain any control outside of Asia and a few islands of the Pacific. Arguably they are the least threatening of the bunch.
-Moonrace: ......... Nothing. Seriously, they're just pacifists on the moon and one of their warriors found a high tech suit, got some friends together, and formed a gang.
-Earth Alliance/ZAFT: At each others' throats in the span of two years, after year one ZAFT managed to create a super weapon strong enough to completely dry out the planet (fortunately this did not happen) while EA has nuke-like weapons that can destroy a base right out under itself. In the second year the EA made an orbital weapon that could blast entire cities away from orbit while ZAFT rebuilt their monster. All of this came out of taught hatred for one another from war-lovers. In other words, nobody was safe.
-ALAWS: 4 years give or take; very similar to the Titans although much more "contained" by simply replacing genocide with autonomous authority fast and hard similar to the ESA/OZ.
Now lets take a look at Vagan:
69 years of attacking, longer than all other gundam antags combined. Their attacks destroyed colonies (like the Titans) and in "contained" incidents (like the ALAWS) with somehow massive amounts of resources (like Zeon/Neo Zeon) and with soldiers bred for hatred against the enemy (like EA and ZAFT) with the inability to negotiate for peace (like UNE/RSA). Take this into account the massive influence they have had (like ESA) not just for Flit's generation, but also the other generations that were formed in these years. Over half a century of open war in space with seemingly both sides waning until Vagan finally launch an assault on Earth, catching countless by surprise. From an "in-universe" stand point, they are the worst kind enemy: The kind that remain constant. At the end of the day, is that not the worst type of enemy one could fight against?
Although I prefer the Goldion Bancrushers.
