When you betray somebody else, you also betray yourself.

The line frames betrayal as a self-inflicted moral injury. Singer’s formulation suggests that disloyalty is not merely a harm done to another person but a rupture in one’s own integrity: to violate trust is to act against the self one claims to be. The quote also implies an inward accounting—conscience, memory, and identity preserve the record of treachery even when external consequences are avoided. In this sense, betrayal becomes self-betrayal because it corrodes character and isolates the betrayer from authentic relationships and from a coherent sense of self.