To be specific, Bahamut was the most powerful of the Terrestrial Avatars - those born of Vana'diel itself. He and the other 4 had entered a pact with the Kuluu in their civil war with the Zilart and be brought down the Celestial Capital Al'Taieu under the sea while also simultaneously transporting it (along with the 5th mother crystal) into another dimension rendering it pretty much inaccessible to all (seemingly) and turning all the remaining zilart in the capital to Qasi spirits.

He doesn't so much hate mankind as he fears the influence of the Emptiness and a possible revival of Promathia. The Zilart, under king Eald'narch were trying to restore Vana'diel to its original state, devoid of emptiness which would allow the Celestial Avatars to once again roam free, as well as the Goddess herself and restore the Whisper of Souls (their psychic link with one another that the Kuluu had lost due to the influence of Emptiness). Only the Zilart and the Gods can survive in Vana'diel's true form so naturally Bahamut & the others were having none of that.


Who knows what his story is in XIV? All we know is that he's utterly f'ing massive (as we can clearly see in Turn 1 looking at just 1 of his wings) and that he's quite likely the reason the Allagan race died out. Whether they died trying to save the world from him or if they simply did something stupid to incur his wrath remains to be seen. I wholly expect turn 10 to be an alliance raid against him as I just can't see a standard 8 man party taking down THE Elder primal. Nael was only given a fraction of his power and was a damned hard fight (I hope he's stronger this time and they keep some element of randomness to him unlike existing boss battles).