The Thin Veneer of Civilization

I've been reading things off and on about the "There probably is no God so don't worry and start living" campaign.
For many years the country of Haiti was tightly ruled by a dictator name Papa Doc Duvalier. After he died, his son took over but was eventually deposed. We moved to the country shortly after his overthrown and experienced five revolutions in the three years we were there. It was a country adrift. There was virtually no functioning government and the rule was might is right. The revolutions were caused by bands of former army or police officers getting a hold of some guns and trying to milk the already financially anemic country out of more money. These gangs would rape, steal, murder, basically do what they wanted until another group got together enough weapons or power to replace them. What happens to the human mind in times of extreme unrest is scary. With no anchor of proper behaviour and manners to hold on to, people can be extremely base, stripped down to survival instincts.
This isn't a scientific point for or against any party in the origins debate but a prediction. If there is no creator then there is no real justification for ethics. If there is no purpose in life, then why should I be concerned about others. My goal would be to propagate my own genetic code by whatever means I can and be the one who survives in the "survival of the fittest" race. What reason would I have to work for the common good? There would be no ethical reason to stop me from doing whatever I can to be the top dog.