Why 'should' atheists be moral?

[robvalue]

Also, get the theist to define what moral means. See if they can come up with a definition that doesn't include God in some way. If they can't, then they are asking you an ill-formed question:

"If God doesnt exist, how can you do what God wants you to do?"

If you point out that this is what they are essentially asking, hopefully they will see they are using definitions which don't make sense for the question to be valid.

Often it's not about "winning" an argument, its just about getting a theist to think for themselves. I realised this question kind of parallels this one, "Now you are an adult and don't have your parents telling you what's right and wrong, how do you decide what is right and wrong?" In many ways, atheism is like "growing up". Sadly some people can't, or won't.