John Gill Commentary 2 Samuel 17

Moreover, Ahithophel said to Absalom
Either at the same time, or quickly after he had given the foregoing advice:

let me now choose out twelve thousand men :
out of those that were with Absalom, which shows their number to be large; and twelve thousand are pitched upon with respect to the twelve tribes of Israel, a thousand from every tribe; Josephus has only ten thousand;

and I will arise and pursue after David this night ;
he took upon him to be general of the army, as well as a counsellor; or this he said to show how confident he was of the success of his counsel, that if Absalom, or any other, should decline the conduct of the army upon it, as a hazardous attempt, he would undertake it himself; or rather it may be, he was not willing that Absalom should go out in person with the army, not so much for his own safety, as lest through his affection for the king he should spare him, when he fell into his hands, or they two should be reconciled; he proposed to do it that night, partly for expedition, no time being to be lost, and partly for the greater surprise of David and his men.

And I will come upon him while he [is] weary
With travelling, and with grief, and when endeavouring to get some rest by sleep, and so surprise him unawares, when not on his guard, and in no posture of defence:

and weak handed ;
while the number of men with him is small, and before the people from different parts can come to his assistance:

and will make him afraid ;
strike terror into him and his then, by surprising them suddenly in the night with such a number of men:

and all the people that [are] with him shall flee ;
one one way, and another another, for their own security, and leave David alone:

and I will smite the king only ;
dispatch him, and let the people flee without pursuing them.

And I will bring back all the people to you
Meaning not the people only that were with David, that he would make them prisoners, and bring them with him; for he before proposed to let them make their escape; but to reduce all Israel to the obedience of Absalom at once, by executing this scheme which he had formed:

the man whom you seek [is] as if all returned ;
meaning David, whom he speaks of contemptibly, and whose life it seems Absalom sought, as well as his crown; and he being dead, it would be all over at once with the people; they would immediately return to their own habitations, and yield obedience to Absalom as the rightful heir and successor; all depended on his death, he intimates:

From which it appears that Abarbinel is wrong in suggesting that Absalom did not design to take away the life of his father, only to secure the kingdom to himself in his father's lifetime, who he understood had disposed of it by his will to Solomon; but here Ahithophel plainly declares the intention of Absalom, nor would he have proposed in plain terms to take away the king's life, had Absalom been averse to it; and it is plain by what follows that the thing was pleasing to him:

[so] all the people shall be in peace ;
both parties coalesce under the government of Absalom, and live peaceably under it, and so an entire end of the war.

And the saying pleased Absalom well, and all elders of Israel . That were of the privy council; for it was a well contrived scheme to surprise David his men by night, in the condition they were in; nothing like seizing an opportunity, and making quick dispatch in executing a villainous design;

only it is strange that Absalom could so easily come into a scheme to take away the life of so indulgent a father, which is only to be accounted for by his ambition; but it is stranger still that the elders of Israel should be pleased with it, and agree to and determine upon the death of a prince, who had so many years ruled them with justice and equity, clemency and mercy; and fought their battles for them, and raised their nation to such a pitch of glory it never had before.

Then said Absalom, call now Hushai the Archite also
For it seems he was not at the council board at this time; whether he was as yet admitted to it is not certain: there is something very remarkable in the providence of God, to incline Absalom to have the opinion of Hushai upon this point, when the counsel of Ahithophel was so universally approved of;

And Hushai also being well known to have been an intimate friend and confident of David's, and not so settled and established in the interest of Absalom, and such a sworn friend of his as Ahithophel was; this can only be ascribed to the will of God, to defeat the counsel of Ahithophel, and the wisdom of divine Providence in blinding the mind of Absalom with respect to his counsel, and inclining it to take the opinion of Hushai:

and let us hear likewise what he saith ;
which he might say without any diffidence about the "counsel" given, but knowing that in the multitude of counsellors there is safety; and Hushai being a wise and good counsellor, he might hope and expect that he would give the same advice, and so strengthen and confirm it.