This might be important to understanding Satou's psyche. Back when Satou brought Shouko to her aunt, I think she was of two minds. On the one hand, she saw it as a convenient way of getting a potential obstacle (Shouko) to leave her alone and stop asking questions. She knew she could either scare Shouko away or use reaction as a way to guilt her into withdrawing.
But I think also that part of her genuinely hoped that Shouko wouldn't withdraw, and would remain her friend, unlike all of the other people who had rejected her in the past. So even though Satou set up Shouko and manipulated her, she probably was also genuinely hurt at the perceived betrayal, no matter how unreasonable that is.
I do wonder, if Shouko had passed Satou's "test" earlier, would she have hesitated about killing her here?
For Satou, this is business as usual. While she did cross a line in a way for some readers, in her own head she simply got rid of another threat to her happiness.
https://dynasty-scans.com/chapters/happy_sugar_life_ch22#19
It marks something special for Satou too, but not in that in crosses a line.
Shouko seemed to occupy a very odd grey area, while everyone else that messed with Satou earlier was explicitly filth. Satou probably considered the way she got her friend to leave her alone merciful compared to how she dealth with others, and even after that, she didn't have a specific way to label her and the face she recalled was that of sadness. Satou didn't hate her, and that was "taken advantage of" in her eyes now. And she'll know that Mitsuboshi wasn't convincing enough once she sees Asashi return, too. After these events, she's lost any grey area she had on the outside world and its people, now she knows she can't hesitate anymore.
And she dirted her hands and brought this person who turned out to be filth inside her little guilded cage, her castle, while Shio was in the next room and very able to hear the sound of what was going on. I guess we have to see if that's something that "belies her love" or not.
I agree with everything you've said, but it's worth taking a half-step back and reflecting on how we're like, "I was fine with all those other murders, but this one was just too much."
In fact, to demonstrate what this one does to my own moral compass, I was kinda hoping she'd get around to Yandere-kun sooner rather than later . . .
....True. But the world feels like a place where literally everyone is crazy and kindness and morality has yet to be properly rewarded in the story, so I can't help but feel like Satou's way is justified if only because it works. Also because that's where the yuri is.