In The Selfish Gene, Richard Dawkins states that nice guys finish first:
In a given single species group of animals there will be "good" and "evil" animals. "Good" guys may share food or perform less desirable tasks within the group, while "evil" guys will selfishly take and hoard food and refuse to help others.
While the Evil animal will win out over the Good animal in the short run of a single generation, Evil behaviour will destroy the group over a longer period of time. That is, if the majority of a group is continuously comprised by selfish, evil individuals, the chances of that group surviving over the long run are reduced.
Thus, there is an evolutionary pressure against being evil because groups where selfishness pervades across generations will be naturally selected out of the gene pool. However, why we all aren't really nice to each other after millions of years of this kind of selection is beyond me.
The Nodeshell Rescue Team