What is the central theme in The Palace Thief?

The central theme in The Palace Thief is moral corruption.  The theme is furthered by the characters of both Sedgewick Bell and Hundert.

Sedgewick Bell is morally corrupt from the beginning.  As a student in Hundert's class, Sedgewick Bell continually lies and cheats.  He is never punished (or even reprimanded) for this moral corruption even when Sedgewick Bell wins the Tournament of Roman History through cheating.  Of course, Sedgewick Bell grows up to be a corrupt businessman.  As the head of a large corporation now, Sedgewick Bell asks for yet another Tournament of Roman History and wins again by cheating.  Sedgewick Bell is never exposed as the liar and cheater he truly is.

Hundert's moral corruption appears through his cowardice at not exposing Sedgewick Bell.  Hundert, Sedgewick Bell's original teacher, knows exactly what kind of a liar and a cheater Sedgewick Bell is.  Hundert is told by his superiors to keep Sedgewick Bell's corruption quiet due to his father's position as a senator.  Hundert could have exposed Sedgewick Bell in either tournament; however, Hundert does not.  His cowardice costs Hundert his self-respect as well as the position of headmaster.

Hundert was once manipulated by a powerful senator into making allowances for his ne’er-do well son, whom Hundert knows to have cheated and whose dishonesty he has never exposed.

The only exit from moral corruption can be seen in Hundert's retirement where he warns others of corruption within the system.