Chapter 1- Cold and Dark.

When the guardians defeated Pitch Black, they did it without Jacks help. But afterwards children become fearless, and many ended up dying. Too quote an Idol of mine, "There is no Light or Dark only power and those too weak to seek it" I feel this works here because there is no good or evil, there is only balance. and with Pitch gone the balance is gone, so Jack steps up and chooses to be the Gaurdians new target. He becomes the new Fear.