Without a moral absolute (ie the Bible) then there is no morality, No morality means you can talk yourself into any atrocity when nothing is "wrong". Imagine an atheist government, they choose their right from wrong based upon arbitrary feels, based mostly on who's in charge, who they're sympathetic to, who they want to prosper and who they wish to destroy. Another atheist government would likely have a completely different set of laws... and those laws would differ according to who and how they applied them (as often commie atheists have people above the law who can do no wrong and people below the law who can have no wrong done to them). So, to answer your question, i have to make some assumptions about which moral code you've chosen to uphold as the metric. Without choosing a moral ruleset, there are no rules and Murder is fine in all circumstances. Now, without trying to be offensive, I know youve rejected the Christian set of morals as you plainly state any chance you get. So youre probably like most agnostic atheists who've adopt a "humanist" code of ethics. Now there are lots of loopholes for you with this moral code, make people you dont like "not human", make fetus/babies not human also, make "racists" not people, etc... and in a stretch you can make them "not a person". So in this world, using your humanist code of loose ethics, the reason Murder is wrong is that it denies an innocent their right to personhood. But then since you went down the road of decreeing "rights" you would have to figure out who gives those rights by the state and how they can be taken away. Protip: if ever asked where rights come from say God, because they cant be taken away, and they never have to be granted in the first place.