it's the symbolism, sister

something something Rayla being willing to sacrifice Callum being synonymous with being “willing to sacrifice everything that [she] is,” in canon, because Callum is that fundamentally not only a part of her but also in many ways representing the Best Parts of her (her ‘big heart,’ “Rayla is a hero, she saves people / You’re a hero,” “I risked losing the best thing I ever had...”) and the fact that Rayla is willing to kill it (herself) alongside / through killing him as a perfect encapsulation of just how Low her self worth and valuing of her own Identity really is

I think there’s a few different layers to pick apart in Kosmo’s lines in 6x05.

The first is that he says it couched within the framing of him becoming timeblind, which as Astrid says, mean he’s experiencing “the past, present, and future as one”. Kosmo’s statements regarding Rayla and Callum’s actions/souls are true for their past (1x01 for Rayla and 2x07/5x08 for Callum) and their yet unseen future (7x09 for both). I’d lean towards his meanings, from an in-universe standpoint, being mostly drawn from the past (“in a moment of mercy…”) even if I expect they’ll be recontextualized as also true in future seasons of the show in a more legitimate way.

Secondly, water in TDP is… complicated. Reflections in water are often linked to shame and struggling with identity (Rayla in 1x05 and S3; Callum in S7). Water/the ocean becomes linked to dark magic, but also love, which can be more positive but not entirely. Rivers/the ocean is also seen as representative of Life in general (1x05, 2x06), specifically the futility of trying to control everything > focusing on what you can control (yourself), but even then that’s linked to identity: “Look at what you can control: yourself. Who are you? What do you stand for?”

I think most notably, though, Kosmo’s quotation about Rayla and her specific ripples is meant to parallel Aaravos’ assertion from some of the TDP short stories. Water represented shame for Rayla, which is an emotion she definitely felt over her choice to spare Marcos, struggling with its fallout even in S7 both internally and socially. Kosmo is basically saying “in a moment of mercy” (shame) “you sent ripples” (another source of shame) for a double whammy.

Despite this, though, Rayla reacts positively to his statement, bolstered by Callum’s approval and support—"Yours is a wondrous heart"—and seems to settle to a place similar to him: “It was hard, but it was the right thing. You know that. I know you do.” It’s worth noting, after all, that at her trial Rayla talks the assassins’ spirits through what the mission and her defection from it did amount to: elves and humans escaping an endless, pointless war, and treating each other with honour and respect again… or at least on the path there, away from a world where one sheds blood (others’ or their own) for ‘justice’.

What Kosmo says, therefore—that her choice(s), or anyone’s, can lead to ultimately unknown ends and chain reactions—isn’t that different in theory from what Rayla says… or Aaravos:

The wisest of the humans looked upon the water. His own reflection smiled back at him, and he dared to imagine what such power would feel like in his own hands, should he be allowed to hold it. Imagine, he thought, if I were more than what I am. With a trembling hand he touched the surface of the water. Ripples spread from his fingertips. […]

He could not see how the slightest touch—a breath in the wind, a spark before a flame, a mere teardrop in the sea—could change the world forever. Such elegant simplicity, such power. For ripples grow: They collide, they churn, they drown. They rise like waves or drag into the darkest depths below. I hope the stars were watching. I hope they saw it: the moment their perfect reflections turned warped and ruined, churned to chaos by the touch of a single human hand.

Rayla (and, less directly but more thematically, Callum) being tied to the same principle that the Ripples in the Water can grow and affect the world/the future in unforeseen ways is kind of package for parcel given that Aaravos foils both of them (currently more so Rayla, but I expect Callum’s foiling to step up its game in arc 3). Down to the fact that, likely, Rayla will continue to predominantly embody the Positive version(s) in contrast to Aaravos’ Negative version(s) of the same concepts in Callum’s life.

In summary: Rayla’s choice to spare Marcos set forth a chain reaction then upheld and added to by every other character in the show, and will ultimately result in all of Xadia being continually radically transformed for the better. The ripples resulting from her moment of mercy are still ongoing, and have not yet reached their harmonious end.