[Alcibiades]

Dr. Boogie: Ozymandius has two reasons for why he does his plan from behind the scenes, his stated one (which I don't remember him referencing in the movie) and his implied one.

The stated reason is that he believes he'll be needed to help guide humanity to a lasting peace and prosperity. If he became the villain, even if he united the world against him he wouldn't be able to do the necessary reconstruction work afterwords. That's why he needs a false threat too. It needs to be something people are afraid of but can't really fight against, if he was the villain people really could send armies against him. And since if he died the threat would be over and everyone would start squabbling again, he'd have to battle back against the armies and then his plan for peace is shot. If he died, without his genius to help the world would immediately go back to war with one another.

But the second, and heavily implied reason is that he wants the world to view him as a great man and not a villain. That's why he projects an image of perfection to everyone. In one sense it's good for his cover, but he clearly is obsessed with going down in history as a famous man for the 'right' reasons. He wants to destroy the current world order and rebuild it, but he wants to be remembered for the rebuilding, not the destruction.