Don't put down to mental illness, that which is coldly calculated evil.
They know exactly what they are doing.
Exactly going back to the Feudal system.
Get the Church in on it, so the Church tells them that the decrees from the King and Nobles are what they must obey because that's what God wants. (God Demands you obey your brutal overlords).
Remember these are people that have no conscience or remorse and there is no evil they will not do.
They will have no problem invoking the name of God to try to sway those who are not strong in the faith (a large amount of those that call themselves believers) into going along and doing what they are told to do.
I'd suggest a read of "A Connecticut Yankee in King Arthur's Court" is a really good idea if you want to get an idea of how the system works.
You can get the book for free from a large number of places, or get the audiobook for free from LibreVox