Kürten was designated by Johan to keep Nina Fortner at Heidelberg Castle until he arrived, with the promise that he would pay a lot of money. When Dr. Tenma showed up, he tried to kill him but Nina stopped him and asked Tenma for something to tie the man up with (the tie Eva gave to him). Both Nina and Tenma escaped and, while his hands were tied up with the necktie and he was unable to move, an unseen figure (presumably Johan) murdered him in cold blood right after the two had departed. (Source: Naoki Urasawa's Monster Wiki)