Man, that's a deep cut into the Evillious lore. It's easy to get lost in the seven sins and the massive cast, but if you ask me, the central struggle is a metaphysical one: the concept of 'sin' itself versus the desire for a clean slate. The whole narrative is a machine designed by the God of Chaos to manifest and concentrate sin into physical vessels, the 'Evillious' figures, as a way to eventually purge it from the world through a massive, cyclical apocalypse. That's the framework. But the real, human conflict that drives the plot is the clash between those trapped by their inherited or manufactured sins—like Allen and Elluka being doomed by the Michaela sin—and those like Gallerian and Banica who actively embrace and wield sin as a source of power. It's not a good vs. evil fight; it's fate vs. free will, determinism vs. rebellion, all filtered through this bizarre demonic-clockwork cosmology.
You see this play out in the personal tragedies. Riliane wants to protect her kingdom but is corrupted by the inherited sin of Pride, leading to atrocities. Kayo wants to escape her abusive life but gets twisted by Envy into becoming a killer. Their individual conflicts are microcosms of the larger one: they're all pieces on a board set up by the Gods, fighting against a destiny written into the world's laws. The main plot conflict is whether this cycle of sin and punishment can ever be broken, or if everyone is just a character in a story already written by a capricious deity. For all the grandeur, it often comes down to characters screaming into a void they're only beginning to understand.