Something about Luz and Belos being foils because Luz gives everyone a chance to change, and Belos doesn't. Something about Luz trying to sympathize with Philip even after he betrays her in the past ("I know you want to go home, but..."), versus something about Belos not even wanting to "redeem" the Isles, but eradicate them.
Something about how there's no way to change that will make you "acceptable" to fascists, no way to change enough to be happy in the Emperor's Coven, because it's a system built on wiping you out. Something about how it is possible to better yourself and find a place of belonging among the outcasts, something about spurning the covens and finding happiness in the home of a criminal.
Something about Amity, especially Lilith, and especially Hunter rejecting the system they once perpetuated because it wanted them dead anyway, something about them giving up on trying not to be "wild" or "savage." Finding a community by accepting forgiveness from Luz. By putting their heart into being better people and stopping the harmful cycles they once perpetuated.
Something about Belos — the root of all the bigotry and fascism in the Boiling Isles, albeit not the root in the human realm — never once taking an opportunity to change. Not the one given by Caleb, and surely not the one given by Luz in Elsewhere and Elsewhen, because of course he wouldn't — as a fascist, he doesn't believe in redemption. Of course he always stood for retribution, not restoration. He'll offer chances to Luz and Luz alone, every so often, but in the same breath, he'll also call her crazy and too far gone.
(Something about how Luz is always willing to entertain the possibility of people changing for the better, from Amity to Kikimora to the Collector. How by the end of the series, she gets almost a little too good at recognizing where she's hurt people, and driven to despair because of it; a little too dedicated to making up for her mistakes. How she's really nothing like Belos; how in fact, before she begins to heal, she's unlike him almost to a fault.)
We have something about how claiming that certain people with certain beliefs can never change, and in fact don't "deserve" to change, is equivalent to absolving them of any responsibility to change and be better. How that just isn't a good way to look at the world. How that thinking, at its extremes, can even seal your own fate, shutting down your own capacity to grow as a person.
Something about Puritan concepts of predestination, maybe even about seeing the timeloop Belos became obsessed with as a metaphor for his belief in predestination, standing for the cycle that lead to him needing to die. He wouldn't accept anything less than all witches dying, and in doing so, left the witches with no choice besides letting the rain melt him alive.