Karl Fiers, or the Finisher, as he was better known to his employer and his employer's enemies, was an assassin employed by the facist Communist Red Skull, Albert Malik. So efficient was he at his job, the Finisher only needed a cloth from his intended victim to track them.
Upon learning Richard and Mary Parker were spies in his criminal organization, Malik ordered the Finisher to take care of them. It was he who sabotaged their plane before they went on a mission for the Red Skull (Malik), thus effectively resulting in their deaths.
Years later, Malik would call upon the Finisher again; this time to handle the costumed interloper Spider-Man, who was searching for proof to clear his parents' names (Richard and Mary Parker), whom were branded as traitors to the U.S.
After Malik gave him a piece of torn cloth from Spider-Man's costume, the Finisher set out to take care of his boss' foe once and for all. While in his specially equipped limo and tracking his intended prey, the Finisher launched a missile that locked on to Spider-Man, thanks to a device in the Finisher's limo and the cloth sample. Spider-Man lept away frm the missile's path, and led it straight back to the Finisher's limo, thus effectively killing him.
Before he could die, Spider-Man managed to get a confession out of the Finisher, who detailed his role in the death of Spider-Man's parents, as well as stating that his parents were innocent of all charges.
Spider-Man subsequently found the Red Skull (Malik), and after a second battle, managed to find the physical evidence needed to clear his parents' names.
In the Sinister Six
series of novels written by Adam Troy-Castro, it's revealed Karl had a brother Gustav, who called himself the Gentleman. It was the death of his brother, the Finisher, that prompted him to seek revenge upon Peter Parker, Spider-Man.