apt-bot aside
I've generated a really crazy number of worlds in the latest version, specifically tweaking things here and there to see what will happen. The reason for demons racking up bodacious kill counts is usually that the demon is the last surviving denizen of an outpost that gets attacked a lot. In one memorable game, the sole surviving member of a goblin civ was its demon king, who, despite the kingdom being in ruins, defended against a human civilization all alone for something like 200 years. If you want to see this happen yourself, try cranking up the number of evil squares and civ count as high as they will go (and still successfully make a world!), and you should end up with a cage-match between younger civs.
The demons are able to rack up these incredible killcounts because the worldgen "battle simulator" isn't really done yet. The current system just fights duels between random participants. So... I guess everyone in DF fights single-file in narrow corridors. Anyway; this system usually works out just fine, but when there are large discrepancies in sizes of fighting forces, it starts to seem odd. Hopefully this will be cleaned up sometime in the army arc releases.
I am sadly unable to find a pattern describing why some demons are able to survive so long and others get slaughtered within a few years or rulership. Dragons very frequently "burn up" demons, on account of dragonfire being more fiery than fire (demons should be immune to both fire and magma, but not dragonfire, which is hard-coded and different).