A creature that believed itself to be all-knowing would have no curiosity and no reason to listen to anything it was told.
This wouldn't be so bad if the creature were, in fact, all-knowing.
But it were ignorant, and omnipotent as well in the bargain, the result would be infinite megalomania, eternal tyranny without the possibility of appeal.
Of course, if the creature were omnipotent then all other reality would be sacrificed to its delusion, so that the creature would, in fact, become all-knowing, not because it knew all that existed, but because only what it knew would be allowed to exist.
Better a God who knows it's all imagined, than one who imagines it's all known.