Rational/irrational does not connect necessarily with good and evil. Those are human constructs to allow society to function without so much conflict, but they are implied more towards intention and selflessness vs. selfishness. Rational/irrational has more to do with the best decision with the best outcome, good judgment, etc. but involves more factors than good and evil; it's not just the intended outcome, but also how it effects the individual, his/her peers, and the future as a whole...In a way I suppose it involves good/evil, but it transcends that as well.
Which is why it's a combination of F and T functions; F I would equate more with good/evil, and T I would equate more with cold logical deduction. When you add them together, you get the most reasonable outcome socially and objectively.