She wasn't ever supposed to die. Her society goes rather far out of its way to avoid it.
But there are cost-benefit tradeoffs even against soul-destruction. You could try to put enough parachutes on an airplane for every passenger, in case something goes wrong in midair in a way that makes safe landing impossible but allows enough time for people to put on parachutes and file out the doors. But parachutes have weight, weight costs money to transport, and the exact scenario where all the plane's passengers could evacuate in midair and should evacuate in midair - somebody must have argued it was very very unlikely.
There's enough time before the predictable crash for the passengers to sit and talk and try to come up with some ridiculous way to survive. They're passing over a rocky zone like a shattered mountain range; flat spots for a landing approximately don't exist. The area isn't cold enough for their brains to stay cold and be retrieved, even if they had some way to protect just their own heads from the impact. Five people are frantically working to tie together enough clothing to make an ad-hoc parachute for somebody's four-year-old. It's not going to work, but it's a dignified way to die.
Thellim doesn't consider herself to be either among the few people who should fit into working on the ad-hoc parachute, or the engineers in the far back trying to come up with an impossibly good idea in an impossibly short time. The best she can do is not distract them while they work. Being still and quiet has never been an important part of her private-tradeoff-Virtue before this, but rapid adaptation to altered circumstances is a public-universal-Virtue. She keeps herself still and quiet up until the plane loses enough speed to start diving. She only starts screaming after enough other people are screaming that her screaming won't make any important marginal difference.
The pain is very brief, and then it ends. That's as expected. The part where she's able to note this successful prediction is not as expected.