In the Nezarec's Sin lore tab, Nezarec is referred to as "The Final Form of Death." That's pretty telling with recent evidence. So if the Witness seeks the Final Shape/Form of death, that might mean Nezarec. Nezarec might be a more powerful entity than the Witness himself. Nezarec is not the Darkness however. The Darkness is an advocate of stagnition, not death. We see this in the nature of stasis. Also, the Deep of the Fundament seas was dense and threatened to make all still through its immense gravity. So The Witness and Nezarec work against the Darkness.