Who is responsible for Mr. Pignati's death? Lorraine blames John, and, though John feels guilty, he says that Mr. Pignati would have died anyway. This novel asks us to think about who is ultimately responsible for a bad outcome. Can we be responsible for something we didn't consciously cause? Who is most responsible? The person who started the chain of events? The person who made bad decisions that led to the bad outcome? Can responsibility always be determined? At the heart of John and Lorraine's identities is a whole heap of guilt and equal amounts of blame; we watch them grow up as a result of the guilt they feel and of the blame they cast.
Lorraine and John are responsible for the consequences of their actions, even if those consequences are not intended.
When John says that Mr. Pignati had trespassed and paid with his life (15), he's implying that Mr. Pignati is to blame for his own death.