Murder is wrong because we humans are defining it as wrong. Not all of us, mind you, but enough of us agree that it is a shiity thing to do to make rules against it. Religious or not. There is no ultimate reason why we don't kill, other than most of us are against it. There is no overarching force that defines life as so sacred that only it, that powerful force, can kill whenever it wants but we can't.
The religious can't claim the moral high ground here. How many soldiers have prayed together for a really really good battle before they go out and slaughter their foes? You know, the ones who don't adhere to the ten commandments. There exists no higher force. Nothing "out there" is defining any of this. We humans are.
There have been times in the past when murder was not wrong. Of course, they didn't call it murder. They called it "hunting" or "working for the department of Homeland Security" or something. So folks who eat each other for dinner would be unable to understand your question in the first place, because they probably wouldn't have a word for murder. Because a guys gotta eat.
In other words, you're asking people who know what it means. Which is bound to skew the results.
My reasons for killing or not killing have nothing to do with my lack of a religious stance. I am able to define and accept moral standards without having a god standing over my shoulder; without fearing an infinity in hell or otherwise worrying about irritiating the big guy. I can do that on my own. And frankly, you shouldn't complain. This is to your advantage, the same as having friendly religious neighbors is to your advantage. You get to live long enough to ponder questions like this.
So murder is wrong because most of us humans agree that it is. At least under most circumstances. There are a few of us who say under all circumstances, but most of the rest of us are willing to make an exception here and there. Fans of predator drones and the death penalty and KKK members and vigorously anti-gay bigots with issues can excuse it at times. Gun toting vigilantes who would otherwise not harm others can sometimes justify their innate desire to be judge, jury and executioner.
So there isn't an ultimate reason for not killing. Rather it is a choice of civilized civilizations. Which wouldn't be civilizations if they ran around wiping each other off the face of the planet.
I could go out and kill somebody right now. I don't (and never have) for a variety of reasons. Everything from the golden rule to a strange and unnatural desire not to spend the rest of my life in prison. Plus somewhere along the way I was taught to respect life in other humans. However, I eat cows because for me a ban on killing isn't a universal thing. I could choose not to eat hamburgers and get all uppity and ask why others go around shooting them in the head with a .22 and slicing them up and selling their flesh for a profit, but instead I join in the fray and buy the cheaper cuts of meat happily. It is a choice. One that happens to have fewer instances of bad karma. Yea, I'll have a heart attack or get cancer from eating too many bovine growth hormones or whatever, but just like bodies in car wrecks, we humans accept some forms of death.
But not murder. Usually. Now excuse me while I go stone my neighbor for working last Sunday.