(\(•̀ᵥᵥ•́)/)

To answer the first part of the question, I disagree that it was a positive show of strength, superior foresight, or morality on Kara's end. At best, it ran counter to the rest of the movie, given her lectures to Ruthye about anger and vengeance and all of the Kryptonian scenes emphasizing Kara being tough but still good; and at worst, it displayed a profound rashness considering that Krem's actual misdeeds comprised a sideplot. Kara did not spend the movie being invested in helping the "brides" (trafficking victims) in a way that warranted that much anger and motive at Krem on her part. She was focused on helping Krypto, and that's the main plot. In fact, the movie gives us no reason to believe that Kara would have gone after Krem to help Ruthye (and thus, get roped into helping his other victims) if Krem hadn't stolen Kara's ship and injured Krypto in the process of leaving. So, with all that, why would Kara kill a man to spare the feelings and innocence of a girl she initially refused to help and to assist kidnapped girls she barely paid attention to, to the detriment of her already fragile well-being and in contradiction to her parents' wishes for her?

Following that, the decision to kill Krem doesn't make sense other than as an impulse, which then undermines the persistent narrative that this is an extension of Kara knowing him dying was better than letting him live. A lot of people seem to be of the opinion that letting a villain live is the equivalent of absolving them or failing to hold them accountable. I would disagree with that regardless, but the fact that SGWOT's ending was drastically different demonstrates that there were ways to pivot if properly holding Krem accountable had been the desired route. Kara killing Krem is the obvious result of terrible writing decisions, a general failure to adhere to the source material, and a shitty understanding of the character beyond "angry and cynical."

I know there are other, adjacent narratives: some people claim that Kara killing Krem was meant to be the resolution to her selfishness and myopia — and/or that it was an attempt to retain Ruthye's innocence as aforementioned — but I've yet to see an argument that convincingly makes those cases using the text and subtext of the movie and not just a generous extrapolation of said text. It doesn't constitute "media illiteracy" to believe that the premises, lessons, and plot developments in the movie were poorly executed. There are reasons this movie isn't well-rated that weren't part of the very real onslaught of misogyny Supergirl already faced.

Speaking more generally writ Kara's characterization, I don't see... any version of Kara as a killer. Specifically, I don't see her as someone who can walk away from killing someone without feeling the weight of it, even if she justified it to herself. If the screenwriters wanted me to accept that as fact, then at barest minimum, they needed to give far more weight to Ruthye's plight and to the trafficked "brides" and less to Krypto. The line for Kara's intervention shouldn't have been an impersonal attack on her dog over helping a kid, and the line for her to kill should have been steeper given the wider themes.

To answer the second part of your question, I'm not sure of how it will affect their relationship. However, if there is to be an effect, the writers already missed the best set-up for it. Kal hovers around the periphery of this movie, both literally and in being the more famous, "perennially optimistic" cousin to whom Kara is always compared. It wouldn't have been much effort, in my opinion, for the ending to have included a conversation about what actually happened to Kara — or, in a better movie, a sort of "anti-conversation" in which the reticence speaks for itself. If the DCU is going to continue insist framing them as "opposites," then it should at least do so in ways that matter for reasons that matter.

I still don't like the quote "Clark sees the good in everyone, and I see the truth" for reasons I've explained. I actually like it even less now because Supergirl (2026) also continues the long-standing tradition of ignoring (or missing outright) the subtleties and nuances of Clark's trauma to prop up someone else's, even if just for the moments that Clark is relevant. In the same conversation Kara says the quote, she also says that Kal has a young heart and that he essentially hasn't gone through anything that would harden it. Superman (2025) didn't pre-emptively give the audience a reason to take that as the musings of an unwell or even envious person whose opinion can't be trusted, e.g., by giving Clark complex thoughts on his heritage, flashing to moments he was ostracized, showing his grief, etc.; and Supergirl (2026) needed Kara to be rational and trustworthy enough, even while on a bender, for the audience to believe that her perspective is reasonable and that she is competent. You can see this even in some of the memes about the DCU thus far: Kara will kill you over a dog, and Kal was too upright to kill someone who nearly destroyed the universe as they knew it. Why would anyone doubt Kara's statement? Honestly, it reads as if there was just enough communication between Superman (2025) and Supergirl (2026) to establish a vague continuity and bash Jor-El and Lara with a throw-away line and not much else.

Circling back around, if there is an effect — which I hope there is, lest there be an even worse payoff to Kara killing Krem than there already was — I don't know how the writers would establish it or introduce it comfortably in Man of Tomorrow (2027). I'm of the opinion that they can't let it lie; but Man of Tomorrow is already too crowded, and I'm not interested in the direction the DCU is going so far regarding their characters. I would hate for a potentially pivotal scene in the Supercousins' development to be stripped of its weight with the goal of keeping in continuity with previous movies, and I wasn't aware that could even happen.

There are more knowledgeable fans of Kara ( @karaspal, @kryptonian-kara, @bisupergirl) whose blogs I also suggest visiting if you're wanting more on Kara as a person. At the end of the day, I only really know her in relation to Superman. That doesn't mean I don't care about how she's treated, but they can give you a lot more.