Why did David leave Maria 's home?

In Chapter Five, David is making plans to leave Maria's home, when he overhears Giovanni and Elsa talking about him. Elsa says she will always be grateful to David for saving Maria's life but comments that she's afraid of him, doesn't understand him, is worried about the things he says to Maria about evil being in the world, and how Carlo is becoming more and more unhappy. Giovanni tries to defend him, but after discussing the various mysteries about him, including how well he speaks so many languages, Elsa says she's willing to help in any way she can, pay for his education, clothing, or safety but he cannot stay in the house any longer. Giovanni promises to talk to him the next day, but neither is aware that David has been listening to the entire conversation. He packs up all his belongings, and writes a letter explaining why he is leaving (see "Quotes," p. 128) and saying that he is revealing all he will ever voluntarily reveal.