Kuja is an angel of death, he was supposed to cause wars and kill to power the crystal of life but he hated fallowing orders so he plotted to make himself the greatest power in the world by trying to take over the minds of the summons and use them to his will.
He was successful and tricking the queen into gathering powerful summons some of which were Odin, Atmosos, and Bahamut. Kuja took control over Bahamut and killed the queen. After that Kuja attacked Alexandria but the two summoners of the game summand Alexander the ultimate summon! Kuja tried to take over Alexander but destroyed it and lost Bahamut too.
After that the heroes fallowed him to a hidden world which turned out to be Kuja and the hero's birth place. There Kuja went trance and destroyed the whole place! Sure Sephiroth burnt down a town but Kuja destroyed a whole world with the cast of a single spell!
After that Kuja went to destroy the crystal of life and everything with it. The heroes did kill him but Kuja still destroyed the crystal and left our heroes for dead. But with a trick of fate our heroes lived to fight a GOD!
That pritty much sums it up
[center]
My dragon has issues
()xxxxxx[|]::::::::::::::::::::::::::::::::::::::::::::::>
(+[__]:
WARNING: "Next left" does not mean "turn left immediately."