In some debate between atheists and theists, the theist will often say something along the lines of; "If you don't believe in god, why is murder wrong?" But can't the opposite be argued better? If God does exist why is murder wrong? After all if you murder an innocent no God worth worshiping would send that person to hell just because they didn't believe at the time (or for a more extreme example infanticide; they didn't have the knowledge or capability to believe in God) so then if you murder a child with the intention that this person will go to heaven due to your actions is that moral or immoral? After all, some theists often say that "why be worried about temporary tragedies on this world when infinite pleasure await in the next?" Isn't that instant justification for murder? The child will live out it's entire existence in absolute ecstasy.
Some fanatics would respond to your question with: "Exactly, murder isn't wrong."
Consider cult leaders who ask their followers to commit mass suicide, or reports from Syria of parents sending their children on suicide missions.
However most people who consider themselves religious still consider murder to be a bad thing.
A secular response to this is that religious people subconsciously don't really believe in God, even if superficially they think they do. Freud, in "The Future of an Illusion" argues that religious belief is a neurosis, that belief in the supernatural and in an afterlife is just mankind's neurotic reaction to the fear of nature and death. People fool themselves into believing, but only because deep down inside they don't.
A related idea is discussed in Plato's "Phaedo": Socrates explains why death is not to be feared. Cebes then points out that if this is the case, then there is nothing wrong with suicide. Socrates agrees that there is an apparent inconsistency, but then argues that it is up to God to decide when we die, not ourselves, hence suicide is wrong. Presumably, the same reasoning applies to murder as well.
One might also argue from a strictly utilitarian view point: Killing a child might be good for the child itself, since it will go straight to heaven, but that is offset by the pain and sadness caused to its family, so the net result of murder is still a negative one, and is therefor bad.
I think your question needs more careful statement.
1 "If you don't believe in god, why is murder wrong?" This should either : "If you don't believe in God, why do you believe murder is wrong ?" (keeping the whole question epistemological) or "If there is no God, why is murder wrong ?" (keeping the whole question metaphysical). "If you don't believe in god, why is murder wrong?" runs epistemology and metaphysics together in a way I don't understand.
2 Your counter question, "If God does exist why is murder wrong?" is metaphysical throughout; there is no reference to belief. Right, but you add an assumption about god which is logically unconnected with the question. Theism is only the view that god, or a god of some kind, exists. It does not follow, logically (or theologically), that god has created a heaven for the good or innocent and a hell for the bad or evil. But the idea of murdering an innocent person or child and thus expediting their entry into heaven, so that murder does them a good turn, makes just that assumption. You provide no justification for it.
You've put some thought into the question and it's ingenious in its way but you make an assumption which you do not justify. On whether it can be justified, I make no comment.
According to religions like islam and Christianity which belive in the one and only god, if you kill someone, you will go to hell after you die. So if you kill a person who you are sure he/she is completely innocent, while sending him/her to heaven you're sending yourself to hell. But if the person you've killed wasn't innocent, then you're sending him to hell without giving him a chance to do good things to go to heaven. Hope this helped
It depends on what you mean when you say morality. What decides morality? Is it society? Can you decide for yourself? Is it a greater authority that decides what is right or wrong? Is there no such thing as morality anyway, and thereby a social construct? Is it simply biochemical response to situations that are harmful to survival? Or is morality inherent to creation?
If it is decided by society, you cannot rightfully criticize other cultures because you have no justification for your belief that your culture is better no matter what the other culture does. Revolutionaries in that viewpoint are always immoral because they are fighting against social systems that are always moral no matter what the society is saying.
If you can decide yourself what is right and we each have our own moral truth, you cannot rightfully criticize another person because you are both equally right in your own mind aka Nietzsche's master morality. This type makes it immoral to punish someone by law because although it might have been wrong if the judge did something, it might have been morally correct for the defendant.
If it is a greater authority like God, or a king/emperor that decides what is right and wrong, then it is wrong to criticize or deny their authority on moral matters aka Nietzsche's slave morality. This is one reason a lot of religious people say all atheists are immoral.
If there no such thing as real morality then it is completely illogical to care to criticize anything morally at all and we are all just waiting for death anyway so just do whatever you want. Kill, steal, have sex with the wives/husbands of other people. Everything is fair game in this viewpoint. If you don't want to something you don't have to but under this perspective you don't care if other people do these things.
Biological instinct through evolution does explain why people care about morality if you believe this viewpoint. The fact that we are here would explain our species survival. But it does not explain why survival even matters. Is survival of a species good and why is one species better than another? It comes down to a biological illusion that morality matters at all, a biological construct as opposed to a social construct. In this viewpoint morality doesn't really exist but because everyone else believes it exists due to evolution they will punish you for doing what they think is immoral even though you are not really doing anything wrong.
Finally there's morality as inherent in creation/nature. In this viewpoint it is inherently wrong to cause harm against another's free will. You must believe in free will or in other words the soul. (There is no such thing as free will if we are only a biological machine that both reacts to stimuli and remembers stimuli to react to in a predictable fashion as a series of chemicals that predictably react to the environment.) The soul is necessary for true free will and not just the illusion of free will created by evolutionary biology and social cues that help survival. The concept of the soul is the thing that gives living things inherent value. (The image of God) under this viewpoint, the nature that true choices exist and have value as opposed to chemical reaction to stimulus, means that it is wrong to choose to take another's choices away, especially through suffering because each person has value in this viewpoint. If I don't want to have sex with you, it is wrong to make me. If I don't want you to hit me, it is wrong to hit me. Even if I am unconscious, if the soul exists, you need to hear my soul's choice before you do something to me and therefore must wait until I wake up.
I don't agree with the rest of answers.
Basically, there is a non sequitur fallacy here: the quality of murder is not defined by the existence of God.
All systems of laws (formal, like ethics or justice, and informal, like morals) qualify our actions according to the benefit or prejudice every action produces in order for the group (society) to survive. Murder is good or bad due to it increases the risk of destruction of the group. All systems of law (which includes religious "laws") forbid murder due to such reason.
The existence of God has no part on such judgement.
If you have participated on a debate where the value of murder was assessed according to a group-subjective (subjective to believers) system of rules (religion), you have entered into the fallacy. Murder is not defined by religious principles/laws/gossip/whatever.
If there is no God, religious rules would make no logic, but that doesn't mean we have lost all law systems improving the survival probability. That doesn't mean that we have lost the way of knowing if they are good or bad.
Out of the subject, religious usually prioritize other laws systems rather than theirs. If their book says that they should cut their own arm if it had made them entering into sin, shouldn't they do so? Nope. In such case, morals/formal law is prioritized. No point debating incoherences.