Psychopaths are human beings still, and humans believe in things / have values and principles. In psychopaths those beliefs and values are not informed by feelings of empathy or compassion, but by consequences and understanding, and that tends to make a selfish worldview more likely but not inevitable. It is possible - even if unlikely - for a psychopath to come to the conclusion that values of altruism and cooperation are in their interest because they like living in the kind of world that results from these values being practiced and want to perpetuate it.