Fate is an important theme in The Secret History. It relates to the main characters' love of Ancient Greek literature, where many poems and plays describe the consequences of attempting to escape one's own fate (a fate which is often predicted).
The idea of fate, predestination, or destiny, contrasts with that of free will. If the characters in The Secret History are doomed to murder (or to be murdered, in Bunny's case) to what extent can their actions be described as free will - in other words, to what extent can they be seen as responsible for their crimes? Richard's reliance on the concept of fate can be seen as an attempt by him to escape responsibility for his actions.