Rationality also last I checked doesn't really build ought statements insofar as morality is concerned. You can use it for good or evil and still be rational.
There is no contradiction in being very evil and very rational. It is easy to think of brilliant scientists who were evil beyond evil throughout history as an analogous example.
We know all true scotsmen are good upstanding citizens. If you find a Scotsman who is a criminal, then obviously he is not a true Scotsman.
If you find a rationalist who believes something mad then obviously he is not a true rationalist.
There are now so many logical fallacies that you can point to any argument and say it’s a logical fallacy.