Karl is the overall main antagonist of the entire series.
In 1943, Karl tortured many soldiers in North Africa. One day, Karl ordered the torture of a British agent who was captured on a German battleship. As said agent survived two months of torture, Karl almost believed he was "Death Incarnate". A week after leaving North Africa, Karl ordered the execution of the agent, but found out he escaped. Days later, the agent encountered Karl in Germany, with the intent on killing him, but failed.
Though not seen, Karl tortured B.J. Blazkowicz in Deathshead's headquarters, even scarring his face. According to his notes, Karl seems to be planning an uprising of his own.
It is possible that Karl runs a cloning factory.