The fact is that Necron was deffeated. He would not blew up himself, without any reason. Zidane and the others made him blow up, because they deffeated him. If he is THAT poweful, like some people are saying, he would not be deffeated by Zidane and the others. He would just use his full power to kill them instantlly. But he didn't do it, because he is not THAT powerful. He is not a god.
To say the truth, i belive in the theory of Squall of Seed, that Necron is the core of the Iifa Tree, created by Garland.