So one of the top 5 most important and influential people in the country is upset that they're not the number 1 most important and influential, and they decide that is a good reason to start a murderin'?
Whatever sympathy I feel for her as a child is completely overshadowed by my feelings that this is a good reason why they should not have a hereditary monarchy.
It's not really just about hereditary monarchy, either. This system of ranked classes, which shows one's potential of capabilities, could also be a form of meritocracy in a very divisive, uncontrollable, and unpredictable manner, as one's revelation/gift of class is given by the gods of that world. This means even the most pos person in the royal family, being the last one in line of its generation, could potentially become the crown prince/princess if they possess the highest-ranked class among their siblings.
It's kind of messed up as Camilla was pretty set on taking her role for the crown, and put a lot of her effort into her studies to prepare herself, and has proven she was better than the Iris in multiple subjects. Putting aside right of inheritance by order of birth, that's the kind of merit people should strive for to achieve high-position jobs. Yet despite her efforts, it all became null and void because of the country's admittedly (and quite literal) class-ist royal law prevented her ascension to the throne. We don't know if her father, the king, made that law himself, or how long that law has taken place, but it's clear that there are
very many flaws in it that could pose great danger for not just the royal family, but the citizens of the kingdom, as well.
If the current ruler is unable to change this royal law themselves without garnering support from others in the upper class, then one way or another, a coup with the intention of regime change was bound to happen. That doesn't make Camilla's crime of patricide forgiven, or even justified, but it's understandable.