Why 'should' atheists be moral?

[abaris]

Harming a member of a small family group hurt the group. As we formed larger communities the same rules fit well so we continued to use them.

Also, as I keep repeating over and over, great apes and dogs also have some kind of moral codes and empathy. Recent experiments have shown that much. It probably goes with being a social species and has more to do with evolution than any kind of learned codes of behaviour.