What would be good as a topic about a person who was once successful and famous, but became a catastrophe?

Tragedy is the word that comes to mind.  After all "Oedipus Rex" by Sophocles, which defines tragedy, is a dramatic narrative of a once powerful, successful, and loved king whose life truly becomes a tragedy. 

Since the story of Oedipus, there have been many a powerful man who has committed hamartia, or a tragic mistake as Aristotle defines it in his Poetics.  Upon an examination of political and military figures, often one discovers this "tragic mistake."

Certainly, Richard Nixon is an example of a powerful and successful man who committed tragic mistakes.  Ahead in the polls by eleven points in September of 1960, just two months before the election, Nixon refused to put on TV makeup even though he had been recently ill.  When the cameras recorded his close-ups, his "5 o'clock shadow" showed, he appeared pale, and perspiration on his upper lip from his slight temperature made him appear ill at ease.  A tragic mistake of not ensuring that he would look his best cost Nixon the debates and arguably the election against the handsome, cool John F. Kennedy.

Having lost to Kennedy in 1960 and having narrowly beat Hubert Humphrey in 1968, Nixon became somewhat paranoid in his campaign for re-election in 1972 despite having had a successful first four years as president.  So, he committed his tragic mistake of bugging the Democratic headquarters in the Watergate hotel.

Other presidents, of course, have had their tragic flaws such as Jimmy Carter whose tragic flaw during his administration was his extreme weakness.  His tragic mistake was in not supporting the Shah of Iran, a decision which led to the creation of the theocracy of Iran that is to this day a problem for the U.S. Carter's flaw was that he did not understand that one cannot be a total humanitarian and protect one's nation. His inability to resolve the hostage crisis in Iran after the Shah was disposed was a political debacle and embarrassment to America.

In Reagan's administration, desiring to destroy communism, he went around the wishes of a Democratic Congress which would not fund the sale of arms to the Nicauraguan contras who tried to fight communism.  This scandal implicated numerous members of the Reagan administration as well as the president himself as he denied knoeledge of details of any transactions.