If that were true, we'd all simply be psychopaths, the genes would have long since won. There are important reasons why humans are empathetic, have guilt, concern for others, etc. It is unlikely that psychopaths could ever form a civilization on their own, for instance; one could make a utilitarian argument in favor of behaving the proper way, but without the shortcut of having your brain simply wired to work that way it's going to be an uphill battle.

(Which goes back to one of my drums I beat here with some frequency, which is that while the rationality of humans is often overstated, so is their irrationality... many putatively irrational things like "guilt" or "empathy" in fact exist for reasons, or if you prefer, have significantly more positive effects when considered holistically than a naive analysis might indicate.)