After healing the M’Kraan Crystal, she begins to transition into the Dark Phoenix. Jean begins to question if she can handle her power.
"Her power is a song within her, filling her with a primal glory, an unhuman joy that she's beginning to enjoy more and more" (112).
Mastermind has manipulated Jean to join the Hellfire Club, thirsty for power and strength.
The Dark Phoenix leaves the X-Men behind to find more power and will destroy anything that gets in her way.
"That is what makes humanity virtually unique in the cosmos...this extraordinary capacity for self-sacrifice...Jean Grey could have lived to become a god. But it was more important to her that she die a human."