In answer to your question, "why was Emet Selch and Elidibus so bitter". It's because they have already witnessed the end time once already, what Eorzea is witnessing now with all the people transforming and stuff. They summoned Zodiark to stop they had already lost everything they knew. Eorzea was sort of built on the ashes thousands of years later, after the worlds were split.
In answer to your question, "why was Emet Selch and Elidibus so bitter". It's because they have already witnessed the end time once already, what Eorzea is witnessing now with all the people transforming and stuff. They summoned Zodiark to stop they had already lost everything they knew. Eorzea was sort of built on the ashes thousands of years later, after the worlds were split.