Obviously "black" didn't have the connotations it does for us. But for just that reason to tone down the blackness of the characters would now come over as racist. (When I was at school we were taken to the movie of Othello with Laurence Olivier blacked up as the Moor. That would be considered quite unacceptable now and the RSC only puts on Othello when it has a sufficiently distinguished black actor.)
Aaron has barely more motivation than Iago, given he is a world where violence is so endemic. But he's in a horror movie, whereas Iago starts out as in a domestic romance with exotic touches, which is why mona finds him so shocking. And she probably has finer sensitivity than me in this case.