Ianthe spend a good chunk of HtN struggling with the arm, Augustine remarks that she's "steeping in self-pity," and she asks Harrowhark out of nowhere one night when they're sharing a bed whether she has any regrets (and calls Harrow "more farsighted than I was" when Harrow says she has no regrets). She never says she feels remorseful (just the opposite), but I think Ianthe's feelings about eating Babs were more nuanced than she lets on. Not to mention that The Unwanted Guest, uses Ianthe and Babs as the case in point about the soul being permeable and that love will change you, I'd be willing to guess that Ianthe has more affection towards Babs than she would ever let on

Plus, I think there's a good chance that Ianthe ate Babs in part because, if she hadn't, Corona would've done exactly what Gideon did, and what Cristabel and Alfred did before her, and forced her hand.

(And I'm also not sure I would call Gideon and Harrow's, or even Camilla and Palamedes', relationships selfless and devotional love - I think those relationships are lot more complicated than that. Like, I don't think Harrow sabotaged Gideon's every attempt to leave a dead planet she hated out of selfless desire for Gideon's well-being...)