I thought you were referring to something else, because this speaks better of Sasha than badmouthing her.
She didn't vent her frustrations on Haise out of malice, or because subjecting him to abuse turned her on. But because she was up against a rock and a hard place. Haise's failure to improve meant he could end up dead at any moment. They were taking on increasingly dangerous missions, and that involved ever greater risk... Therefore, she felt ever closer to having to make the decision she DIDN'T want to make, which was to leave him behind, to expel him.
She vented her frustrations on him, firstly, because she wanted him to get better and had to be tough with him to make it happen. And secondly, because she felt frustrated that if he didn't improve, the inevitable would happen: expel him. And that filled her with terror and sadness, like when she said that every time she treated him like that, he cried... but that he would do it again the next day, and the next, until he could stay with them.
You speak as if she did it out of whim, greed, pleasure, or something like that, when the fact that she was frustrated and being harsh with him meant she didn't want to resort to exile... and both the treatment and the idea of expelling him made her cry.
Honestly, I thought you were referring to something else, not this one, which I did remember...
Sasha wasn't a good childhood friend for several reasons, and quite a few (for which she has to apologize sooner or later), but this particular thing you're saying is ridiculous, and it speaks better of her than anything else.
That's much further ahead than what's translated into English, where I'm reading the novel, which is currently on issue 199.
Do I understand that's in Japanese? If so, I'd rather wait for it to be translated than hire a translator.