I guess that Ysayle looked at the Ishgardians as oathbreakers and she knew that somehow Hydaelyn and Midgardsomr are on the same level (hello.... he stripped the Warrior of Light of Hydaelyn's blessing). So, she wanted to get back at the Ishgardians for mistreating Midgardsomr?
Remember that Dravanians are also a race, albeit they're not humanoid, and they live for a very long time. So, if the Warrior of Light stands for Hydaelyn and Eorzea, couldn't Ysayle stand for Midgardsomr and the Dravanians? We fight Garleans because they want to rule over us but the Dravanians are already been betrayed by the Ishgardians (for whatever reason). Now Ysayle wanted revenge against the Ishgardians for the sake of the Dravanians.

Speculations though.