Shrug of God: There is no official answer as to the true nature or motives of the Dark Powers. Any source purporting to give one is by nature non-canon. In the most recent edition, the Dark Powers are officially undefined - that is to say, there is no true explanation for them, and never will be. They are intended to be whatever any given GM wants them to be. The closest fans have ever gotten to an "offical" explanation as to the nature of the Dark Powers is that the Red Death from the Masque Of The Red Death setting is a Dark Power who got exiled for breaking the Dark Powers Code of Conduct.
That, and Vecna does address them briefly and in the plural as he tears his way free of the demiplane, suggesting that they do exist and there's more than one of them. As Vecna is stated to have broken out because he'd seen through their enigmatic facade and learned their true purpose, it's reasonably certain that he's right about this.