Reach the final level in the game in the White House, and choose either
to kill Tom Reed or spare him to get the "Judge, Jury and Executioner"
achievement (if you kill him) or "Man Of Conviction" achievement (if you
spare him). To easily get both achievements, once the game is completed
and the credits have begun, 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 to
get the other achievement.