The primordial serpents seem to have their own agenda with regard to the "Dark Lord" the end of the "Age of Fire" and the "Abyss". My guess is that Frampt is there to lure would be heroes into sacrificing themselves to a pyre that won't ultimately be saved no matter how many you throw in there, and Kaathe is looking for their true Dark Lord to emerge from the endless number of candidates.
Remember that all of Lordran is unstuck in time, with events looping back on themselves endlessly. While probably not responsible for it, the Primordial Serpents are probably capable of exploiting it. They will have their Dark Lord, if they wait long enough, and by the same token, all those noble souls who throw themselves into the Kiln of the First Flame to re-link the Fire do them a favor by rendering themselves incapable of challenging that Lord that they await, and of course the fire is going to go out eventually in any case.
If Kaathe is to be believed, the fire was already going out when Gwyn came down from Anor Londo and linked it the first time, meaning we're at the end of what was already borrowed time. if the Dark Soul manifests, bringing about the Age of Darkness/Humanity that Kaathe talks about, it is easy to believe that the Abyss, whatever that actually entails, will be involved, and while it might not be where they come from, the serpents definitely have access to it.
