At some point, Dean became the bearer of the Mark of Cain and was converted into a Knight of Hell. This led to a fight with Sam in the Men of Letters bunker where Dean beat Sam up and threw him through the railing of the upper level to the floor below. Despite Sam's pleas to stop, Dean continued to throw his brother around before strangling him up against the wall one-handed. Drawing the First Blade, Dean stabbed Sam in the side, killing him. Dean is now sitting in the bunker drinking beer