The Assignment by Dan Gordon

Carlos the Jackal is a terrorist without rules, without a conscience, without a heart. And no intelligence agency in the free world has been able to stop him. CIA agent Jack Shaw has seen what Carlos is capable of. Now he wants Naval Lt. Commander Annibal Ramirez to help him. Ramirez has a nice wife, two kids, a house in the suburbs...and Carlos' face. All Shaw has to do is train Ramirez to act like Carlos and kill him.

Ramirez is Carlos' exact double, Shaw's pawn in a chilling game to trap the terrorist. And stripped of his morals, family ties, and his soul, Ramirez becomes his mirror image--a ruthless psychopath on a collision course with pure evil--where only one man will survive.