He is the sovereign Lord!

2 Samuel 17:1-23
Absalom and all the men of Israel said, “The advice of Hushai the Arkite is better than that of Ahithophel.” For the Lord had determined to frustrate the good advice of Ahithophel in order to bring disaster on Absalom.
Ahithophel's advice is to strike quickly, and only against the person of king- so avoiding civil war. But Hushai gains time for David by a scheme which appeals to Absalom's vanity虛榮.
Ahithophel has the foresight to realize the probable consequences - hence his suicide.
Why?
The writer points the message. The will of God works through human beings in the detail: David will be restored.
How?
Yes, the will of God works through human beings, why should we worry about! My hope is in hands of God! He is the sovereign Lord!