Law ask Corazon since when he could talk, and Corazon tells him he could always talk.
He never said he can't talk, Dofla just took upon himself to believe so.
Law yells and accuses him for deception, and Corazon uses "Silence" to put a sound-proof barrier between them and the town, erasing all sound coming from the town. (or going out)
Law says was you being a clumsy also an act?
Corazon says of course... but sets himself on fire with the cigarett accidentally.
It seems he's a natural klutz.
His motive is to stop his brother from going out of control.
He can't understand why such a monster was born from the kind hearted parents they had.
Doflamingo isn't a human..
Flashback
The Donquixote family are pinned to the wall, and the father cries out for mercy on his children.
But Doflamingo tells the mob below, that he will not die here. He will live, and kill every single one of you!
Corazon explains only he and the other Executive officers, as well as the previous Corazon, "Vergo", know of Dofla's true insidious nature...