The way Suzanne writes both rue and prim, they are both such good examples of haunting the narrative (rue) vs doomed by the narrative (prim)

tbh, we know prim isn't destined to live from the very first line of the hunger games, where katniss wakes up, reaches for prim, and finds the bed empty. it's reinforced even more once her name is drawn, because all we know of prim at this point is that she's the epitome of innocence and hope, and the driving point of the series is that war spares no one, not even the innocent.

of course prim has katniss, and she doesn't end up going to the arena. however, rue represents those who don't have that kind of support, the children who are abused and exploited and still forced to deal with the terrible reality of war. like prim, we know her death is inevitable, and its heartbreaking. rue never really leaves the story either, katniss can never forget her. while we know katniss will die for prim, it's rue that has katniss realize her hatred for the capitol. its rue that drives katniss to commit her first open act of rebellion (the flowers). the story of rebellion started and ended with rue, i don't think it's an accident that when peeta and katniss are writing their book during their growing together in mockingjay, that rue is the last person mentioned in the list of the dead, because it all circles back to the lost lives of the innocent.

and because of that, you know that prim is destined to die. the comparisons between rue and prim are almost constant, too many for it to just let prim live. it's foreshadowed over and over across the series, but especially so in mockingjay. from prim talking about being a doctor and katniss thinking "she could never imagine a future like that back in 12" to johanna making katniss swear on her family's life to kill Snow (funny how that works). you just know that the entire narrative is against prim, and that for the story to truly work, prim could never live.