Loghain and Saren are both not in their right minds. Only, Saren has an excuse (indoctrination/implants) and a legitimate fear driving him (the Reapers should easily be able to destroy the galactic civilization). Moreover, it seems as though his plan was on target: the Reapers *will* use organics as tools.
In Saren's mind, the option is either become a slave to a powerful, ancient evil or just die. That is a rational assessment of the situation for those not blessed to be the Player wish fulfillment Character.
Loghain is just nuts. He takes a strong and unified kingdom, starts a civil war, lets the Blight ravage the countryside, and allows enslavement of the populous, all because he