Rejoining question SPOILERS

[Anonymoose]

From the way the NPCs walk about it, the primordial forces pool around their agents like gravity wells. Just as what we call Light is drawn to the agents of Hydaelyn, Darkness is drawn to the Ascians. Killing an agent of Hydaelyn or Zodiark causes a sudden shift in the balance - and any shift sudden and/or violent enough will trigger a Flood.

The First was already Light-heavy, but had fallen into a delicate homeostasis. The Shadowkeeper was supposed to help break that homeostasis and tilt a bit farther towards Light (the path of least resistance, I suppose), which would pull all of the elements to an unnaturally umbral charge and cause it to begin to diffuse into the Source. Striking down the Warriors of Light while triggering a disaster on the Source would have led to a Rejoining. Instead, the Warriors of Light defeated and spared the Shadowkeeper and killed her masters, causing a violent lurch even farther towards the Light.