Why did Tyuule feel sad? In the final episode Tyuule said to herself that she did what she did to get revenge for her fallen country. But what was that made her sad? The actions that she took, or that she fell in love with the prince Zolzal?
A bit late, but it's because all revenge does is leave empty. So even through Tyuule got what she wanted, she can't feel happy about any of it. They mention it earlier back with Tuka's mental issues and the fire dragon.
Oh and she doesn't love him. Can never love him for all he's done.