Raven: Charles, what did you do?
Professor Charles Xavier: I had to keep her stable. I protected her.
Raven: From the truth? There's another word for that.
Professor Charles Xavier: She'll kill us all.
Jean Grey: They're right to fear me.
Smith: You're special, Jean, and if you stop fighting that force inside you, f you embrace it, you will possess the very power of a God.
Erik Lehnsherr: The world is on the brink.
Professor Charles Xavier: I'm sorry, I didn't stop this sooner.
Erik Lehnsherr: You're always sorry, Charles. And there's always a speech. But nobody cares.
Ororo Munroe: Sometimes, you want to believe that people are something that they are not. By the time you realise who they are, it's too late.
Young Jean Grey: They say you can fix me?
Professor Charles Xavier: Jean, you are not broken.
Smith: You're special, Jean. And if you stop fighting that force inside you, if you embrace it, you will possess the very power of a god.
Erik Lehnsherr: I've seen evil... and I'm looking at it now.
Jean Grey: I'm scared. And when I lose control, bad things happen to the people I love.
Scott Summers: Tell me how to fix this, Charles! Tell me what to do.
Professor Charles Xavier: I don't know what to do.
Scott Summers: Jean lost control. But she's still our friend.
Answer: Your guess is correct. Lawrence stated she was done with the franchise after "X-Men: Apocalypse." She jokingly told Simon Kinberg she would return for this film if he directed it, and to her surprise, he was serious about it and held her to it. Fans weren't very keen on her appearances either, because she was definitely phoning it in for the third film, so that may have factored into killing her off early as well.
Phaneron ★
More than one actress had played Mystique. Just because Jennifer Lawrence did not want to continue playing that role is not a reason the character would be killed off. They could easily recast another notable actress in the part. I would not be surprised if Mystique is miraculously "resurrected" in a future X-Men film with a new actress.
raywest ★
Aside from Deadpool, any new X-Men film would be a complete overhaul of the franchise since the characters will be part of the MCU now. You're not wrong about an actor wanting out to not really be a reason to kill off a character, but it doesn't preclude them from doing so either. They might have decided it wouldn't be worth the hassle of recasting the role with all the negative baggage that would come with it.
Phaneron ★
Here are some candidates I think could do it: Amanda Seyfried, Shailene Woodley, Chloe Grace Moretz. Heck maybe even Milla Jovovich-Anderson should be given an audition, what say you guys?
Rob245