Karl and his brother Tony joined Hans in his plot to rob $640 million worth of bearer bonds from the Nakatomi Plaza. When Tony was killed by John McClane, Karl became enraged and wanted revenge. Multiple times through the film, Karl and the other terrorists attempt to kill McClane but always manage to fail.
Near the end of the film, Karl confronts a badly injured McClane one last time. The two fight bare-handed and their struggle leads them towards the roof of the building. McClane shouts at him while punching him that he should heard his brother's "squeal when he broke his fucking neck". Karl gave McClane several kicks on McClane's face and shot McClane in the shoulder with the pistol. However, McClane wrapped a chain around Karl's neck and left him hanging by a chain as the roof exploded and everybody fled.
After Hans was killed and the authorities started evacuating everyone, Karl managed to break free from the chain and went down to the first floor with the hostages. As people were walking out of the building, he disguised himself as a hostage and drew his Steyr AUG in a last ditch effort to try to kill John and Holly. However, Sgt. Al Powell finally got the nerve to draw his gun to save John and shot Karl dead. Even though he kept getting hit by revolver bullets, Karl was so blind with pure rage that he barely flinched, still aiming his assault rifle at John, only to be stopped by a swift gunshot to the heart.