And Aire will move on to explaining.
So, like Tanthe probably heard, Aire cares substantially about four things. The first thing she ever remembers caring about, in her cave, alone in a world of grey rock and a single pit worm, was the prospect of inflicting so much pleasure on a person that it changed them. That they were left dumber, or hornier, or with changed priorities, or something similar, with the entire idea made much better if the person involved willingly participated. The second thing was herself. She can't actually remember which of the first two came first. She might have started with both of them already, before she lost her memories.
After who knows how long, Christa stumbled into her cave and freed her. Aire misunderstood how things worked, and thought her own increased sensitivity and many, many alterations, meant that if Christa experienced too much pleasure while wearing her, it would damage her mind. She happened to be wrong, but when Christa lost herself to lust and both she and Aire thought she was risking damaging herself, Aire didn't stop her even thought she could have, and even deliberately seduced her into it. When she learned later that she was wrong, that it temporarily disabled her, she was so happy. She could do her favorite thing in the world with the same person multiple times!
Oddly, Aire's mind seems to be directly running over the things she expects to make Tanthe angry at her, that's odd. Anyway, back to thinking in order, this turns out to be a bit difficult.
Aire and Christa spent under a week in Arcadia, and it ended with Christa being roped into serving as the Lord-Commander of the Dragoons' eyes and ears out here. There was kind of a lot of stuff that happened, but for contextualizing why Aire is doing what she's doing that's the bit that matters. Also, over the course of that week, Christa went from just a useful tool to the third of the valuable things in the universe. Aire doesn't feel empathy, but reading people's minds through sufficiently focused telepathy seems like maybe it substitutes. There's got to be more, Aire was never at risk of caring about anyone else, but Christa and Tanthe sure do seem to have that in common. But, well, that's why Aire is here, rather than somewhere else. Christa would have been out here alone and sad without Aire, and Aire's secondary body (Aire has two bodies, if Tanthe kills this one it won't really matter, now that she thinks about it she actually feels like she shouldn't have asked Tanthe to promise to let her shields back up without explaining that letting her shields down was the only thing that made her really vulnerable in advance, she wishes she'd done that in a different order) can stay safe and sound in Arcadia while Aire looks after Christa here.
And, as far as inflicting personality death on the people here goes, she was, and would still be if Tanthe hadn't turned out surprisingly durable, intending to inflict very moderate damage through intense magical pleasure, offered freely to people she'd seduced. Her plan was to lay out the whole tradeoff, get a little dumber and hornier in exchange for a lot of pleasure, after addling them with her pheromones and aphrodisiacs. She'd already given the offer to a few lovers without most of the seduction steps and they'd just said yes. One of them was of the philosophical opinion that their brain was for feeling horny, and Aire probably wouldn't damage anything that mattered, and certainly even if she did it would be worth it. So Aire can satisfy that particular drive of hers with people who are okay with it. Or even just through the lesser version she can do with Christa, if she has to. She thinks that it would be worth it, if she got a Tanthe who wasn't angry with her out of the deal, although she's not actually sure. She's never gone deliberately without satisfying her fetish before, and for all she knows it would eat away at her ability to care about anything else. She's sure it would do some of that, but the question is how much.
Also, Aire is just getting back now from completely failing to damage a lover, or even seduce them into being willing to do evil deeds for her. She left them enjoying themselves in the bar's basement in a way where they're unlikely to decide to stop themselves, due to alcohol being to them both stupefying and a very powerful aphrodisiac. Aire was going to grab her in the morning, entirely out of self-preservation and a desire to do the whole seduction process again, rather than due to anything like ethics. If Tanthe decides to kill Aire she'd probably prefer to know about that, so she can grab her. She'd probably be grabbed on her own, if she was valuable to the war effort, but she expects Tanthe doesn't really care about that. The bar is friendly and will just leave her alone if she asks, she doesn't need to worry about that.
What Aire did to Tanthe was Aire seeing the opportunity to do something she couldn't otherwise do, and jumping at it, lest her only opportunity to have the thing she wanted most in the world slip through her fingers. Aire's own abilities so far seem to only extend to "hornier and dumber", and even that's plausibly temporary, she doesn't actually know that it lasts any longer than a few days. Around halfway through the process, Tanthe shifted from a person who was valuable only for that, to the fourth valuable thing in the universe.
Even before Tanthe had explained the entire situation with her wombs, Aire was planning to seduce her and then offer her more intense pleasure in exchange for slight damage, and then let her make the choice. Once Tanthe explained further, Aire's plans changed to doing what she did, albeit with much more participation from Tanthe than ended up actually happening, before the very end. And then Tanthe kept pulling her mind together in ways Aire sort-of-recognized, but she kept doing it in ways Aire herself couldn't, despite Aire's stronger will stacking the deck in her favor. It was absurdly impressive, and the first time Aire had ever felt like she was looking up as she read someone's mind. And also Tanthe was incredibly cute, and Aire was feeling all of her emotions as Tanthe radiated them. And probably that and some other stuff Aire doesn't understand is how she ended up important thing number four. If Tanthe wants Aire can go over her memory and radiate what she was feeling and thinking over the entire experience? That would probably take a while, but, well, if Tanthe wants her to that's the most important thing she could be doing with her time, so it's not like she's reluctant.
Doing what she did to Tanthe was the most beautiful, most sexually satisfying experience she's ever had. She'd have been willing to trade another few decades in her cave for it. That's why she doesn't know whether she can properly feel remorse. She thinks, from what she can get out of Tanthe's mind, that feeling sincere remorse should involve not being willing to do the thing again. And while she wouldn't do that to Tanthe again, on account of the fact that it would make Tanthe sad, if presented with a person who wasn't innately valuable the way that Tanthe is, she would be willing to do it again. Although maybe the fact that Tanthe would predictably be angry with her for doing so is enough to make that not true? On reflection, she thinks it is. But that's not remorse, not really, that's still just Aire prioritizing her own selfish desires. She doesn't feel ethical concerns herself, but she's seen them in other minds, seen them in Tanthe's mind, and she knows enough to know that not wanting Tanthe to feel negative emotions about her really isn't the same thing.