Ciri has a very strong sense of responsibility and duty. It resonates throughout the game. If you try and talk her down from facing the White Frost in the end, something that won’t even affect humanity in anyone’s lifespan, you get rebuked and with a massive put-down to boot. “What do you know silly, you’re but a simple Witcher”.I personally thought the empress ending was horrifying for Ciri. I get the idea, she comes into a position at the heart of the most powerful nation in the world and can change things for the better from the inside. But there's no way that would be a happy life for Ciri. She has spent her entire life wanting to be free and living her own life away from all those people who have a plan for her.
As empress her entire life would be pretty much written in stone for her, it's about as free as a lifetime prison sentence. Not to mention she'd have to go live with one of the most horrible individuals in the witcher universe, a man who has caused innumerable suffering and death across the world and pretty much made her life a living hell for so long.
A witcher life would give her the freedom she's always wanted. Sure it's dangerous, but for her, a very skilled fighter and incredibly powerful source, it's arguably far, far less so than a life among the politics and scheming at the heart of Nilfgaard, where she'd face a constant threat of assassination and plotting against her for the rest of her life. She can easily deal with danger she can see coming as a witcher, less so with the various hidden dangers as empress.
And if she gets tired of the witchering and solitude, she can just go live with Geralt and Yen for as long as she wants. It's a no brainer really.
She can save lives at risk to her own, so she chooses to. She always takes the tough option to help others. Similarly as Empress she has the option to make the lives of so many people so much better. So she chooses to.
Emphasis on chooses. Cause y’all are forgetting that after she has a talk with her dad in Vizima, she chooses to assume that responsibility. She still has the option to fake her death after fighting Wild Hunt/White Frost and slipping into the shadows to become a Witcher, but she does not.