Serial Killer X, also known by his real name, Leland Vanhorn, is one of the main antagonists of the game Condemned: Criminal Origins.
It is PewDiePie's main objective to find and catch Vanhorn, although this proves to be quite difficult.
Near the end of the game, PewDie/Thomas (main protagonist of the game) discovers Serial Killer X and battles him with the spear of his recent victim, but Vanhorn's uncle cheap shots him and knocks him out in an attempt to talk down his nephew- which fails. When he wakes up, Vanhorn is there and cuts one of his fingers off before trying to kill him, but is attacked and subdued by his uncle before he can do anything.
Despite the fact that PewDie failed to record the ending of Condemned, Vanhorn is known to have been tied up in the trunk of his uncle's car and put at the mercy of Thomas. Whether or not PewDie would have chosen to execute Vanhorn with a shot to the head or spare him is unknown, but would not matter regardless- as he would shoot himself in the head even if PewDie saw fit to spare his life.