imrightuarewrong

You were going pretty good until the last two paragraphs, then you just basically tried to label atheists as murderers and went over the deep end. And what god are you referring to exactly when you say "your god"?

And of course right and wrong are relative. You're telling me that if your family is starving and the only way you can get food to eat so that you don't die is by stealing it, you would still deem that "wrong" and not do it?

Muder is "wrong fundamentally" because it's not beneficial. The advantages of being in a social group outweigh the advantages obtained by murdering someone else in that social group. If you murder someone, you're gone from the group and lose all of those benefits and you have to fend for yourself.

I like how your original question is "why is murder fundamentally wrong" but in your last sentence you restate it to be "why not do what's best for yourself" implying that murder is somehow the best thing for an individual. That is clearly not the case.