To say that someone did something because he is a madman is to confess that we cannot really explain it at all. This is the fundamental insight, and the methodological point, behind the rationality principle.
The rationality principle is not the empirical hypothesis that each person acts adequately to the situation. That hypothesis is clearly false. It is, on the contrary, a methodological principle that places restrictions on what will and will not count as a rational explanation. It says that if we want to explain a social event rationally, then we must assume that the people in it acted adequately to the situation, or, at the very least, that they acted adequately to the situation as they saw it.