Atheists, explain why murder is immoral philosophically?

Basically golden rule has the answer. You are a tiny part of the system and when you do murder, you bring an act of murder into the system which means that eventually you can get murdered too. This system (society) is not something separate that we live in: being part of this system, we shape it by our own acts/ideas/etc. And that's why it is wrong to murder :D Well, at least I see it that way.