The arena scene was simply padding
I saw the movie last night and was mostly impressed, just as I was with Part One. Yet I couldn't help thinking that Feyd-Rautha's black-and-white arena scene was just padding and didn't really add anything to the movie. Irulan has already described him as a psychopath and we've seen that all the Harkonnens are heartless, bloodthirsty killers. (Aside: how do they get so many victims to be part of their personal staff/entourages? Would you want to work for them?) I suppose the scene could be simply preparation for his final fight with Paul, but we've already been shown his prowess as a fighter.
It's been decades since I've read the book, so if this is taken directly from it, I duly apologize to all the Frank Herbert fans. But the filmmakers obviously made a number of changes (especially to Chani's role), so why would they add this if it's not in the book? Other than giving Austin Butler even more opportunities to glower into the camera, the only thing I can think of would be his discovery that one of his victims has not been drugged, which informs the viewers yet once again how duplicitous the Harkonnens are.