Reason, and if you can evidence. Prone to like Machiavelli more nowadays. Rather than to be nice, and lose.
The problem with this is that fearfulness feels bad and love feels good. Being moral means not deliberately hurting others. There is nothing in the long run to gain from this. If we life a life of stomping all over others and grabbing as much stuff as we can for ourselves, it does not result in happiness or inner peace.
A life in which we respect ourselves and others, even if we die penniless, is a life well spent. Look at the history of the world and its wicked leaders. They killed and trampled over people for their own glory. But how much glory does Hitler, Stalin, Mao, or any of history’s mass murderers enjoy? Their names will go down in infamy for all recorded time.
It is the Law of the Jungle to kill or be killed. But we are not living in a jungle. We are humans who owe it to ourselves and others to co-exist in peace. If we follow our baser instincts we will merely postpone the next phase in our evolution.