And all men shall fear, and shall declare the work of God. The fate of David's enemies shall cause widespread fear and alarm. Men shall perceive God's hand in it, and shall be led, in consequence, to declare "God's work."
The tragic ends of Ahithophel and Absalom were certainly well calculated to impress the minds of men generally, and to strike awe into the hearts of those who had looked with indifference, or even, perhaps, with satisfaction, on the political troubles.
For they shall wisely consider of his doing; rather, for they shall understand his operation. They shall understand, i.e; that God is on the side of the righteous, and, when danger presses, will interpose on their behalf, to the terrible discomfiture of the wicked.