Why are you so worked up over what he said? It wasn't even funny (tho I've never really found Foxx to be my sort of comedy tea), and it's a Tarantino movie at that...you'll find worst in his own films than what actors in them say off the set.
Personally, Foxx's comment just falls into a line part of a culture of race-based jokes, which are tasteful if done right, balanced out and not relied on the mere shock of them. But a lot of so-called "comedians" think if you put <ethnic group> + "people" in the same sentence and laugh your way through, it'll be funny. No, not unless there's substance there, and a real joke too.
I think Foxx was trying to be funny when saying it, but as a joke it fell a bit flat. In the context of the movie tho, it's fitting ('cuz that's essentially what his character does, to put it rashly).
I think we aughta also be more careful when using the term "racist" too, tho it's really something for a whole different time. Cut to the chase, Foxx made a joke as a black guy about killing white people. Joke isn't funny, but only b/c his delivery sucked. When you say 'racist' you make it sound like he had malicious intent in saying it; clearly he did not. But it's to a point in this country now where you can't even talk about someone of a different skin tone w/o being called a racist by others, and that's especially sad.