Let me now choose out twelve thousand men - Had this counsel been followed, David and his little troop would soon have been destroyed; nothing but the miraculous interposition of God could have saved them. Twelve thousand chosen troops coming against him, in his totally unprepared state, would have soon settled the business of the kingdom. Ahithophel well saw that, this advice neglected, all was lost.
This night - The night of the day on which David fled, and Absalom entered into Jerusalem. Ahithophel‘s idea was to fall upon David by surprise, and in the first confusion of the surprised army to seize and kill David only.