Board Thread:Dark Souls II/@comment-10209824-20140506193921/@comment-16047389-20140507135854

There are some problems with this theory.

First off, to rekindle the First Flame is to ultimately be sacrificed to it. Gwyn linked the First Flame and was reduced to Cinder, basically Hollowed, as well as being locked away in the Kiln for the rest of his life. The Chosen Undead suffers the same fate if they choose to link the First Flame. If the Lost Sinner attempted to do it, she would not have survived the process.

Second, the souls placed in the Lordvessel aren't used to rekindle the First Flame, they were only used in Lordran to open the path to the Kiln of the First Flame. Once again, the actually rekindling is the sacrfice of the soul of the one attempting to link the First Flame. We know this, because even if you choose to let the First Flame burn out in the first game, you still have to use the Great Souls to open the path. The Lordvessel only acts as a key, not an actual offering.

The idea that the Lost Sinner is the Witch of Izalith makes more sense in the grand scheme, and why here sin would be so great. The Witch of Izalith, in her attempts to recreate the First Flame, created the Bed of Chaos, Chaos Pyromancy, and unleashed Demons upon the world.