The standard atheist way of defining morality is what advances human flourishing. This is their starting point, and they have no grounding for it; this is their standard but they have no place from which to hang it. It is simply the way it is - if something contributes to human flourishing, it's good; if it doesn't then it's bad.
I hope you can see what this means - it means that, on their view, the only thing that is good is human flourishing, actions such as rape, theft, assault, murder etc. are not evil in themselves - they are just not conducive to human flourishing (although that point could be argued, in that, on evolution, the weak would and should be eliminated, and the DNA of the strongest should advance - after all, it is survival of the fittest!)
This removes the morality or immorality from the action - the crimes listed above aren't intrinsically wrong, they just don't serve to meet this goal of human flourishing. This really reduces these actions to the level of driving on the wrong side of the road - the actions are forbidden and seen to be wrong just because of the consequences to society. But this is will not do. We know that child abuse and torture are wrong in themselves, irrespective of what social consequences they have. Atheism cannot account for this, neither can it adequately explain why we feel guilt over things that no one else knows and that will never be discovered. Christianity can explain it - there is a righteous God who has made us in His image, and He has impressed the demand of His law on our hearts (Romans 2 v 15).
The answer to our guilt is not for us to deny or excuse it, but to admit it, and receive the forgiveness offered by God on the basis of the sacrifice of Christ (Acts 13 v 38).
