i felt like it was their destiny to kill her in that lifetime since the FL said it was like a fairytale where the Hero had to kill the Demon King to live happily ever after. Although they don't want to do it, it's just how her life ends in that turn.
and you're also right. the author should give contexts why they had to kill her or whatnot she's too good for these people fr
I'm guessing their a victim to the exact same thing the mc has faced in her countless lives, no matter how hard she tried it'll always end the exact same way all 100 of her lives ending in her inevitable death before rhe age of 20, there seems to be some type of external power as seen but the white haired child and the magical phone, I wish it went more in depth tho but thats as far as I can theorize

I still didn't understand why they had to kill her at the end in the FL being demon king life
I dont like the fact she forgave so easily
I mean author should had made this more clear why they killed her not just because the story goes like this
She was not in a pre determined novel nor was someone controlling their actions since the princess could run away from her home easily so many times to the FL's castle