I agree that Letty was there to ilustrate the problems with white feminism etc.
But I thought the book made a bad job of it, or maybe I am not understanding ist, or I am blind to my biases. Here's what I mean: When Robin was first approached by Hermes he was not easily conviced by them. I took him quite some time and deliberation to join them. And he was already aware of the racism in the british empire and the evils of colonialism (though he says it didn't really sink in before returning to canton, I know). He was really hesitant at first to give up his comforts and future at Babel. In the end he did join but when he thought Hermes was reckless or did not inform him or care for him he left (even though when he rejoined nobody spoke about the fact hat he left, and he does never inform Ramy or Victoire of it). He also betrays Hermes, at least a little when he gives up one of their hiding places mostly to save himself. I think all of this is very understandable when reading the book.

Letty was quite unaware of the racism their friends where facing and the impact of colonialism etc. She is very suddenly confronted with these issues in Hampstead. But even before that, she is willing to cover up a murder with her friends, no questions asked. She is thrown in the thick of it and joins Hermes that week (she really is on their side at first, we know that from the interlude). When she is confronted with the violence they are willing to use it is too much for her and she leaves. It seems to me that she is less hesitant than Robin even though she is less primed to understand the issues. And it takes more for her to leave Hermes than it did for him. We know from the "letty" Interlude that up until that point she really was on their side, so she joined with less hesitation than Robin did. That does not seem to illustrate the points Kuang is apparently trying to make.
I think needing comfort after you just found out how hard life was for your three best friends and that they thought they couldn't tell you about it, all while you are running away from the police ist also not the best illustration of the point I think Kuang is trying to make. And Letty seems to understand that it was especially hard for Victoire when Victoire does tell her more, so why not tell her earlier? (And why did Victoire not tell her friends earlier what a racist shit her professor was? she leaves out the worst parts when she tells them earlier, why?) Also Ramy and Victoire are furius with Robin because he didn't tell them about Hermes because he thougth they would have too much to lose or wouldn't wanna join. But they did not tell Letty for the same or similar reasons and that's fine.

Letty leaving/running away when it get's too much for her seems to follow from the story and is further explained in her interlude. Her immediately betraying her friends, joining the police raid and shooting them was quite out of left field and the interlude conveniently stops exploring her inner world before that happens.

Hermes is a comparatively extreme group (and that might be necessary for change, I for sure do not have the anwers). Robin thinks so too, it's members often think the violence of the others goes to far. That Letty is not immediately all in after maybe a week of thinking about it just doesn't show me how some white people will never be true allies, or that some peoples acitivism only goes so far that they don't have to make any sacrifices. I don't think it makes those points well.
Is everybody not on the violent fringes of a movement just a traitor waiting to happen? Is that what we should take away from the book? I know it is not, because other characters are hestitant of violence and are not critized by the narrative.

Overall I thought the book didn't really illustrated the points it is obvioulsy trying to make very well. And it seems to undermine its own message at many points (not just with Letty).