I felt it devalued the game because for 2/3 of the game you are fighting for freedom. Then, in the last act, you suddenly find out about this huge evil thing your brother was trying to stop by himself being evil. It took the emotion out of trying to overthrow him because now you see his motivations in the right place he just didn't know what to do in order to save everyone.

It was also devalued because "the great evil" was a lame enemy that posed no real threat. If you bought houses and shops you could earn the money to defeat the evil in no time. The entire point of making choices based on gold was silly because you should have had piles of it by that point of the game. So your promises (to keep them or not) really meant nothing.

Had it taken you 10-15 hours to become King, then another 10-15 hours as you find out about this evil threat and try to defeat it while running a country I think the game would have been better served.

As to the Fable 2 ending, I don't think either of them were good. I don't think I can even call them endings because of how bad they were.