"Red Dead Redemption" has special gameplay for when you accept a challenge to a duel. Once the "Draw!" command is given, everything will go into slow motion and you'll have a few seconds to aim your shots before the game returns to normal speed. In some duels, you can choose to disarm your opponent instead of killing them, but when a duel is part of a story mission, like in the case of Harold Thornton, your only option is to kill your opponent.
Play through the game until you get the quest called "The Wronged Woman." Go into the town of Blackwater. Enter the bar, where Harold Thornton will be standing near the counter.
Talk to Thornton and he'll challenge you to a duel. Accept his challenge and go outside. The duel will begin a few seconds after you get outside.
Press and hold the "L2" button for PlayStation 3 or left trigger button for Xbox 360 when you see the word "Draw!" appear on the screen. Use the right joystick to aim the target at the head of Thornton.
Press the "R1" button for PlayStation 3 or the right bumper button for Xbox 360 repeatedly until the blue bar on the bottom right corner of the screen fills up. Press the appropriate button when the target is white, as that indicates a good shot, while a red target indicates a bad shot. When the bar is full, you'll go out of the slow motion dueling mode and your bullets will all fly at his head. This will kill him and win you the duel.
Tips & Warnings
- You can also get a cheap win in this duel by shooting Harold Thornton as soon as he steps outside, before the official duel begins. This will put a bounty on your head and you'll lose "Honor" points, but it will fulfill the requirements of the mission.