Gitta Sereny, the daughter of a Hungarian aristocrat who dedicated herself to explaining in writing the monstrous acts of individuals from a concentration camp commander to a child murderer, has died. She was 91. The veteran journalist and writer built a reputation as a fearless pursuer of uncomfortable truths with a series of books that confronted the authors of some of the most reviled acts of the 20th century and in so doing became a controversial figure herself, in particular for her decision to pay the child killer Mary Bell for co-operating on a biography about her crimes.