Man's possession of free will is what allows him to make moral choices. Being selfish does not mean sacrificing others to oneself. This is just a variation on the theme of sacrifice. To be rationally selfish one must neither sacrifice himself to others, nor sacrifice others to himself. The morality of rational self-interest sees each person as an end in himself, not a means to the ends of others.
Altruism sees each individual as the means to the ends of others.
In other words, it sees people as sacrificial animals. Altruism is an invalid code of morality because it is destructive toward human life. This can be seen by thinking about what would happen to a person if he attempted to practice altruism consistently. His own death would be the result. For example, when deciding whether he should eat a plate of food in front of him, altruism would tell him to sacrifice himself and give the food to someone who needs it more.