Was talking with a friend earlier today, and even though it's an optional choice in gameplay, the thought of Diamant and Ivy both having to kill their fathers is so painfully... symbolic?
Both Morion and Hyacinth represent the old order, in a sense. They uphold the cycle of war and hatred that's held Brodia and Elusia in a grip for at least thirty years, as innocents of both countries are caught in the crossfire, their lives taken and villages razed. Neither of these kings (or the leaders who came before them) have been willing to compromise.
And after all this time, as hatred and fear has settled into the fabric of both societies, it's not so easy to do.
Brodia invades Elusia to claim territory, but also cite the country's worship of the Fell Dragon as a major reason. I definitely think that, for some of the more ambitious Brodians (particularly the nobility, often painted as a problem in Diamant's supports), it's a convenient excuse. Both Firene and Solm are more tolerant in Elusia's choice of worship, and not even Lumera has raised a hand to stop them after all this time.
But Morion, portrayed as a hero in the eyes of the Brodian people, and an honorable warrior (to his eventual downfall), gives me the sense that he genuinely buys into the dangers of the Fell religion, and believes that the invasions are the best way to protect Brodia in the long run.
Hyacinth, portrayed by Hortensia and Ivy as a once kind man, is implied to have turned to extreme forms of Fell worship to combat the Brodian threat, i.e. resurrecting Sombron and being caught under his thrall. This, of course, leads to the Elusian army being used for the Fell Dragon's ambitions, and plunging all of Elyos into chaos.
Brodian society honors might and claiming things by force, while Diamant wants to reform it into a peaceful country whose economy doesn't hinge on war.
Elusia worships the Fell Dragon, while Ivy turned to worshipping the Divine as an act of rebellion, and because their values aligned with her own. (Or at least, the type of person she wished to be.)
By being the ones to end their fathers, it's another way of symbolically cutting their ties with that old order, and clearing the way for a better future they for Brodia and Elusia. One that they give each other the courage to achieve, and work together to make a reality.
But it no doubt hurts, because despite their faults, you can tell they both loved their fathers deeply. :")
