Rational Self interest is different from your perceived Irrational Self interest. If you think A is not A then you are not rational.
When one speaks of man's right to exist for his own sake, for his own rational self-interest, most people assume automatically that this means his right to sacrifice others. Such an assumption is a confession of their own belief that to injure, enslave, rob or murder others is in man's self-interest—which he must selflessly renounce. The idea that man's self-interest can be served only by a non-sacrificial relationship with others has never occurred to those humanitarian apostles of unselfishness, who proclaim their desire to achieve the brotherhood of men. And it will not occur to them, or to anyone, so long as the concept “rational” is omitted from the context of “values,” “desires,” “self-interest” and ethics.
“The Objectivist Ethics,” The Virtue of Selfishness, 30.
You have to think to understand.