There's a difference between depictions of violence, and depictions of cruelty. I don't think I've ever seen a protracted depiction of cruelty in a comedy.
People can get murdered in all sorts of funny ways, but there isn't that element of induced suffering. And when there is, it's not funny (broadly speaking, of course, humour is subjective).
Out of interest, do you think the content of this scene could have been made humorous? I always got the impression that Tarantino was trying to lighten it up as much as possible, but the callousness sort of mutes any potential comedy.