Because murder is a convenient point to ponder because we can pretty much all (except for maybe dictators and drug lords) agree that it is wrong.
Until people agree on something, they can't even begin to address the "why" behind it.
ETA: Kind of funny, but I could have easily replaced "murder" with "homosexuality" in Kingston's arguments and it would have been exactly as reasonable. In fact, it wouldn't surprise me that if you took the entirety of history, murder is much more "normative" than homosexuality. So much for "normativity"!
