Offended by Tropic Thunder?

Why in the world would anyone find the "retard" scene in Tropic Thunder more offensive than say, the killing of people or a panda bear? And to an extent, wasn't Robert Downey Jr.'s character right in his ysis of why Tugg Speedman didn't get any recognition for Simple Jack? (I hope I got Stiller's character right, and the movie he was in)