If God is a made up entity existing nowhere but our own minds, then of course "God says so" is arbitrary. However, in the question that you're asking, He's not. The question "Why is murder bad under Christianity" can be restated as "IF, assuming, presupposing, let it be for a moment, suppose that it really is the case that the Christian God IS..., THEN why is murder wrong?" And of course the answer is that the Christian God defines morality, and murder can quite succintly be wrong because He says so.
Here is where it falls apart for you. You say that God defines morality and that He also says murder is bad. But what if we can point to you MANY instances in the very word of God (the bible) where God specifically condones, encourages and joyfully partakes in mass murder? And what about all the times in history where one group takes up arms against another in war, and God does nothing to stop it? How can you make an objective claim that murder is bad when your God partakes of it on da' regular, and chooses not to intervene when war breaks out? Can you not point to specific points in the bible where murder is happily condoned? Of course you can.
More succinctly, if a man points a gun at you and you say, "Don't kill me because God says it's wrong", if the man says "God condones murder on massive scales in the bible", what will you say to him? Or what if he says "If God doesn't want you to die, He will make my gun misfire." Are you willing to take your chances? Or what if he said, "God spoke to me like he spoke to Jephthah. He told me to kill you." What fun...
Here is a clear cut explanation as to how we KNOW you don't get your morality from God. Do you believe that everything that God does in the bible is morally right? I doubt it. If you did, then you would happily smash children into rocks, kill homosexuals, destroy the town I live in, etc. So in essence, your morality is JUST as subjective as ours, yet you assert that it's not because SOME of the book agrees with you. You pick and choose those passages out of the bible that back up what what YOU think is morally correct, and then you discard the rest. Why do you do that? Why would you do that if morality was objective? Isn't the bible the very word of God? Or are you honestly going to say that YOUR interpretation of the bible is correct in every way and supported 100% by all the passages in it? How can you possibly stand on that premise with the bible as your source? Unless you are going to point to another source (non-bible) for where you get your morality, it's laughable.
And worse yet, which version of your God should we believe in? Yours? The bible is so vague as it is possible to theologically defend the idea of slavery, homophobia, genocide etc etc. Maybe we should adopt the morality of the WBC, who scream out God hates Fags and protest soldier funerals? Or should we adhere to the Catholicism of the Nazi regime and the current version that says it is better to stop the use of condoms even if AIDS is the result? Or maybe the Christian God in the eyes of the KKK? Or maybe we should start killing people who don't keep the Sabbath. Whether you like it or not, all of those positions are THEOLOGICALLY DEFENSIBLE positions from the bible. You CAN defend them using biblical passages. Yet somehow, you think YOU have it right that murder is objectively bad?
This is why the Christian version of morality is VASTLY more dangerous than atheism. Because some of the most horrible things that men do to other men are done as a result of a specific type of interpretation of the bible. Why were heretics burned at the stake for hundreds of years? The bible. Why were women thought of as second class citizens for centuries? The bible. Why do people bomb abortion clinics? The bible. Why does the WBC protest soldier funerals? The bible. Yes, atheist leaders have done terrible things, but that only goes to show that all people can be bad... Christian or not. Christianity just gives some bad people a way to justify the bad things they do. What is more dangerous than a mistaken belief that a supernatural sky man is asking someone to do something for the glory of God?
Subjective morality has its bad points too. I acknowledge them. I don't even like it all that much. But it's simply the way it is.
Two materialists are chatting, and one of them is bragging about his embezzlement of $10,000,000 from his employer. The theft is so perfect there's no evidence. The other is shocked, of course, but I contend that he can't give a good reason, using just materialism, why the first ought to be condemned.
Two Christians are chatting, and one of them is bragging about his embezzlement of $10,000,000 from his employer. The theft is so perfect there's no evidence. The other is shocked, of course, and he can appeal to Christianity to explain why the first ought to beg for forgiveness from his employer.
In both cases, the man who stole the money thought it was right, and the other thought it was wrong. That is evidence that morality is subjective, is it not? Otherwise all of them would have thought it wrong.
You can appeal all you want to your sky man. The FACT is, this man will get away with the crime if neither of the 2 turn him in. The FACT is, both of them thought differently about the crime. The FACT is God will not punish him in any provable way at all. If we follow the facts, where do they lead? Do they lead to the idea of an objective source of morality or one that acknowledges that everyone has different versions?
This all stems from your belief in God MiC. You want the world to be a certain way. You want there to be objective morality, because it fits with your version of theism. We get that. That doesn't make it true. Truth doesn't care what we want. Stop looking at what you believe and what you want, and start observing the facts. Accepting where the facts lead is a grown-up thing to do. Nobody can point to an objective set of rules or laws that govern our morality for everyone. They just aren't there. But we can explain how our morality got there, and what benefits we get out of it. And that's all we really need to know. You believe
your morals are grounded in objectivity... but if that doesn't fit with the facts, why do you think it's still logical to hold that position?