A theme is "the central and dominating idea in a literary work" (eNotes). With this in mind, we can say that the dominating idea of Stephen King's novella Apt Pupil is the human capacity for evil. From Dussander's Nazi war crimes to Todd's infatuation of them and later adoption of his own murderous activities, King's work seems to speak to the inherent evil of mankind. With Dussander's retelling of the atrocities he committed during the Holocaust, something base seemed to be unlocked within Todd's mind. He turned to violence to relieve himself of his nightmares. And after many years spent lying low, Dussander begins to do the same.
Their actions, sparked by the oral history Dussander is forced to relay to Todd, lead to King's apparent conclusion that violence breeds violence.