what are we supposed to say again when christians ask us where we get our morality?

[Tonus]

But we may consider that moral, depending on the circumstances. Society may feel that killing a person who is a direct threat to you or your loved ones is a moral act. Or, uh... killing someone who was collecting firewood on the Sabbath.

…so according to your definition of morality both killings are morally good acts?

They would be moral to the members of the society that defined them as such. I would not call them good, as I would not personally think that it's good to kill a person in either circumstance.

There are some actions that I find difficult to argue as not being inherently bad, such as rape. But most are going to be conditional, even if we consider them wrong in nearly every other circumstance. The wealthy company owner who cheats his employees out of a fair wage is doing something wrong. The man who steals a loaf of bread to feed his starving family will probably be seen in a different light.

In a purely material Universe why would any act be inherently bad? Only because some people find them distasteful?

My point is that few actions can be judged independent of context. Perhaps there are no actions that could be objectively labeled as bad. I pointed out one action that I cannot make conform to that idea.