In a dialog, Robert Wright says to Frans de Waal, a primatologist (transcribed from here, 33:07):
Robert Wright: “If somebody says to you, ‘Look, there’s no God, so what’s wrong with me just killing this person? You tell me, why is it wrong to make someone else suffer? If it makes me better off by making someone suffer, what’s wrong with that?’, what do you say to them?”
Frans de Waal: “Well, I would say, ‘I, as a representative of the community, I don’t agree with that kind of behaviour, because next time it’s going to be me, or next time it’s going to be my family, and so I have good reasons, selfish reasons, to object to this kind of behaviour which we don’t tolerate in this community, and we will punish you for that.’ And I think that’s how morality came into being, communities with a single voice would tell you how they felt you needed to behave.”
In other words, according to de Waal if one can do something ‘wrong’ without the community catching one, then there’s no negative consequence to doing something wrong (however that is defined), i.e., it is rational to do the wrong thing when not doing it conflicts with self-interest and if one can get away with it.
So we can rephrase the question put to de Waal as an anonymous note slipped into de Waal’s mailbox by our hypothetical bad-doer:
“I’m going to do something you and the rest of the community object to (call ‘wrong’), but I’m going to do it in a way that I won’t be caught. Can you give me a good reason not to do it?”
Based on what de Waal says in the dialog, the answer is: “no.”
Yet, the whole point of morality is to make it so that it doesn’t matter if one is caught, which is why de Waal’s reasoning seems like a giant step backward. What de Waal is proposing, rather, is called legislation, police, and courts.
This can be contrasted with a more traditional kind of Christian morality. In that kind of case, there is no way to get away with it. The moral laws aren’t so much things that can be broken as things that one can break oneself against. This is because doing something wrong leads to estrangement from God (say), which in itself is a bad thing (either in this world or afterwards, through Hell or Heaven).
In this case, when our anonymous hypothetical bad-doer asks: “Can you give me a good reason not to do this bad thing?” the person can say “Because you will be worse off for it, in this world or the next.”