If you get to the final level in the game in the White House and choose either to kill Tom Reed or spare him, you will receive the achievement "Judge, Jury and Executioner" (if you kill him), or the "Man Of Conviction" achievement (if you spare him). To get both achievements easily, once the game has finished and the credits have started, select the "Continue Story" option at the main menu. You will resume the game closer to the ending choice, instead of repeating the entire game again, and can select the other ending.