Some of my classmates chose to go with an "intuitionist" argument, ie "Everyone feels it's wrong, so it must be." That doesn't cut it. Not only have there been societies which condone murder but there have been societies that feel eating pork is a grave sin. So the intuitionist argument is a poor one.
Like I've said, I don't see how it is possible to prove that murder is wrong. Even if one believes in God you can always ask, "Well, why should I listen to God or think His opinion is better than mine?" It's a slippery slope.
B.