Sorry, but you’re wrong on this one. People have complained about minorities being cast in traditionally white roles multiple times. “But Nick Fury isn’t black! Why are they casting a black person there?”
You can look at all of these and think back to at least a few times when people got upset at a black person being cast in a role they thought should go to a white: https://www.ranker.com/list/black-actors-who-played-white-characters/lisa-waugh
I never even heard of the Murder on the Orient Express movie. I have no idea how much attention it got, but it looks like the movie veered from the novel in many more ways than just having a black character: http://www.slate.com/blogs/browbeat/2017/11/10/murder_on_the_orient_express_movie_vs_book_comparison.html
I never read the book, but that says Leslie Odom Jr.'s character is a composite of two different ones from the book. It even mentions the following:
The movie is hardly colorblind about its casting. In the book, characters are especially suspicious of Antonio Foscarelli, because they believe that a knife is an Italian’s weapon of choice. In the movie, that suspicion shifts toward Arbuthnot and Marquez, as Poirot worries that the crime could be pinned on one of them, guilty or not, because of their respective race and heritage. Poirot also comments on the challenges facing the movie’s interracial couple, Arbuthnot and Mary Debenham (Daisy Ridley).
I have no idea if their skin color or ethnicity was ever specified in the book, and I honestly don’t care. I just watched a movie called Death Note in the past couple days and I had no idea it was based on Japanese manga until I looked up some info about it and saw that there was a backlash about the story using Americanized characters and names instead of Japanese, but even the creators of the original work praised and defended the movie.
Anyway, I’m not buying your white guilt excuse.