Doctor Mendelhaus, called “The Butcher of Diebenwald” was a Nazi doctor who experimented and killed plenty Jewish people sent there. He worked in Diebenwald, a Nazi concentration camp deep in the heart of Germany.
He was responsible for Anna Kapplebaum‘s parents death.
When Nazi forces where defeated at WWII he escaped to America. He reappeared when he entered into a butcher shop and Anna Kapplebaum saw him.
Being an old man he disagreed with his past. He told Ms. Kapplebaum that he saw Satan (ref. Hitler). Klaus was kidnapped, along with Anna Kapplebaum, by new Nazi followers. They required him to help them to re-establish a new Nazi regime but Captain America defeated all of them.
Klaus was killed by Marie Heller as an act of vengeance for his father’s death. With his last word Klaus remembered Anna Kapplebaum.