The more rational someone is, the less you can trust them. The less rational someone is, the more you can trust them.

This is not always the case. In fact, in practice I trust rational people far more than those that are irrational. If I know what a rational person's goals are, know the respective payoffs for each of the various available options then I can reliably predict what the rational person will do. Irrational people can not always be counted on to do what is in their own best interest - this makes them less predictable than they would be if they were rational unless they are an adversary trying specifically to foil your intelligence.