Ah...oversimplified. If I remember the LN correctly she was evil, but not that evil. The hero party removed the opposing factions, just as mentioned and everyone hoped, that a new golden age would dawn, but... The opposin forces also contributed a lot the country's strength. Not only a huge part of the military joined her uncle, but also a big part of the economic wing and other rather important factions. After everything was said and done the country was weakend, some neighbors stopped trading, as their contacts have been (literary) cut off and other neighbors started to move in hostile ways. First came economic collaps and then war. The hero's party heard about it a few years later and have been shocked and saddened about the news. They mused about the bad state the world is in. (All being the fault of the demon king, of course.) In short the princess bit off more she could chew. The uncle on the other side, while power hungry was indeed capable. (And I believe he was her brother instead of her uncle in the LN, but I might confuse something here.)