Ahithophel hanged himself for vexation that his counsel was not followed. That will break a proud man's heart which will not break a humble man's sleep. He thought himself in danger, concluding, that, because his counsel was not followed, Absalom's cause would fail; and to prevent a possible public execution, he does justice upon himself.
Thus the breath is stopped, and the head laid low, from which nothing could be expected but mischief. Absalom chased his father. But observe how God sometimes makes up to his people that comfort from strangers, which they are disappointed of in their own families.