What prevents an atheist from committing murder

Why don't atheists commit murder, if there's no accountability in the afterlife?

Commentor Ben raised an interesting point about atheism and moral law:

The fact is, of course, that ordinary atheists are decent ethical people, just as ordinary Christians, Jews and Muslims are decent people. I think that the evidence is overwhelming that state atheism is invariably violent, and there may be evidence that Christians are statistically "nicer" people (giving to charity, etc). But on a personal day-to-day basis, atheists are just as nice as anyone.

The atheist/materialist explanation for this homogenous niceness is that we are evolved to be moral. The Christian explanation is that the Moral Law is written in our hearts by God. And I don't believe that fear of eternal retribution is what keeps most people from doing horrible things. Most of us wouldn't kill even if we were sure we would never be punished.

My issue with atheism and  the moral law is a different issue. This is it:

If atheism/mateialism is true, than the moral law is not objectively true. It's subjectively true. It's something we made ourselves, not something pressing in on us from without.

I disagree with the atheist view of the moral law.  I believe that murder is objectively immoral, independent of opinion. I believe that it's objectively wrong, even if everyone in the world believed otherwise.

So are atheists really willing to accept the logical conclusion of their belief: there is no objective moral law, that murder is not wrong in itself, but is merely wrong because we say so, and we could, at some point, say differently?

Are there atheists who do believe in objective Moral Law? If so, whence the objectivity?

My suspicion is that this is something that atheists don't want to think about with much rigor.

What do atheists believe?