The premise that you start with here that:
“it is in one’s best interest to do what best benefits oneself.”
is false, or at least incomplete.
One can act according to one’s best interest only so long as you do not effect violence or threat of violence upon another in doing so. This qualification is an essential part of rational self interest.
If you think about it, a society in which everyone is free to commit acts of aggression against each other in pursuit of self interest makes life safe for no-one in the community. Therefore a complete ability to act in self-interest without the boundary of the non-aggression principle is, in fact, not in one’s self interest.