I mean, the fact that she WILL kill at least one of our protagonists because otherwise her plans can't be fulfilled should be enough.
Like, she IS going to kill Kagura, no IFs or BUTs about it. If only because of that last bit with the dragon daughter, where even if her ideals or way of life were something she could accept (they aren't, but the way she denies them kinda implies her dedication to her goal would just ignore them regardless if she was also a vessel instead), she would still go through whoever antagonizes her ideals...or is an obstacle to them.
Unless you don't care about Kagura (and, by extension, Raito), Cattleya is terrifying as an antagonist, if not for her strength, because there's no real reason to oppose her besides fearing her world might be less gentle due to how strict she is with her ideals, or how bad things could get if she gets possessed.
In the best case scenario, our protagonists have to fail their own goal of getting to live together while exploring the world because otherwise they're postponing Cattleya's remaking of it in a way that doesn't promote so much suffering, which can be extrapolated in an extreme way as them willingly continue the pain of millions of people for no other reason than to have some semblance of happiness together for that amount of time. And a lot of people, if they stand behind her goal, would immediately turn on the both of them with such reasoning should she want to argue in such a way.
Essentially, imagine a Grifith after making the pact with the Godhand, but actually used their powers purely for good (or at least some convincing interpretation of "universal ethical good") and just acts, instead of only the selfish satisfaction of their own desires. With the same capacity to enchant and bring people together because of the ideals they spout and some amount of charisma, and certainly a comparable amount of power, but still having to kill Guts and Casca because otherwise they can't actually fully wield the power they were granted (and they aren't a sociopath that would torture one to mess with the other).
Would Cattleya leave her for last? Probably, the help wouldn't be bad, and if not that, she wouldn't want to kill Kagura until she is forced to...but she would still kill Kagura all the same, as soon as that's the last thing left to do, because of the everything I've said already.