Ahithophel אחיתופל

His measure is given in one line, without qualification: "the counsel Ahithophel gave in those days was as if one inquired of the word of God; such was all Ahithophel's counsel, both to David and to Absalom."

Why he went over to Absalom the text does not explain. The genealogies give room for a guess — by one reconstruction Bathsheba was his granddaughter — but this is nowhere stated outright, and the guess remains a guess.

His counsel was professionally flawless: give me twelve thousand men, I will set out this very night, overtake the king while he is weary and weak, throw him alone into confusion, and bring all the people back to you whole. No massacre, no long war — one precise blow.

Against him spoke Hushai, sent by David for exactly this: your father is a soldier, he does not sleep among the troops, he is already in a pit; gather all Israel and lead them yourself. The counsel was worse but sounded weightier — and Absalom chose it. The text comments directly: so the LORD ordained to defeat the good counsel of Ahithophel.

The rest is three lines: "Ahithophel saw that his counsel had not been followed, saddled his donkey, rose and went to his house, to his town, and set his house in order, and hanged himself." He understood how the revolt would end before anyone else did.

The Mishnah (Sanhedrin 10:2) names him among the four commoners who have no share in the world to come.