Why 'should' atheists be moral?

[LostLocke]

Religious people often ask the question if you are an atheist why do you not go around murdering people

I think the real question is, why would I go around murdering people?

By their question, they are saying that our default position is to want to run around murdering people, but their god puts the brakes on that by issuing 'morals' that we have to live by.
In other words, they are indirectly implying that if they learned their god didn't exist, or their god declared that life was now a free-for-all, they would turn into murderers.