I think a more interesting topic is, why do religionists always seem to ask, "Why is murder wrong?" when they've decided to teach the rest of us a lesson on morals. I think it's a strange question, because the definition of murder is (from MW):
1 : the crime of unlawfully killing a person especially with malice aforethought
The answer to the religionist's question then is simply, "By definition. The word is defined as wrongful or unlawful killing". It's trivial. There is nothing to think about.
Oh I know, what they really mean is, in what way and in what circumstances is killing unlawful? How is the law defined etc? Well, they do mean that, and they don't. If they really meant that, once in a while they'd ask that question, but they never do. They choose to ask, instead, "Why is a polygon with three sides a triangle?"
My belief is that they are so fully indoctrinated with the idea that right and wrong are intrinsic to a theistic world that they can't even - some of them after many years of discussing the matter - figure out how to wrap their heads around the right questions that need to be asked. Morality is so given to them, it's akin to a well-known conversation from Huxley's Crome Yellow,
Rowley turned at last, slowly and ponderously and nobly, as he did everything, and addressed himself to Henry Wimbush.
"Look at them, sir," he said, with a motion of his hand towards the wallowing swine. "Rightly is they called pigs."
"Rightly indeed," Mr. Wimbush agreed.
"I am abashed by that man," said Mr. Scogan, as old Rowley plodded off slowly and with dignity. "What wisdom, what judgment, what a sense of values! 'Rightly are they called swine.' Yes. And I wish I could, with as much justice, say, 'Rightly are we called men.'"
Old farm-hand Rowley and the originator of the MAD thread (who is officially writing on behalf of FAIR) are equally helpless for similar reasons. And will equally get the unreflecting shouts of approval from their peers, "Yeah, murder sure done is wrong, how d'you explain that one? How can you say them there filthy swine should be called anything but pigs?"