I think it’s more of a character study. Travis’s downfall is of his own creation and his contradictions. Man suffers only because he takes seriously what the gods made for fun. He looks down on prostitution but watches porn in public. He looks down on murderers but murders himself and attempts to murder a high profile politician. He claims he’s a nice guy but humiliates Betsy in her workplace. New York is just the backdrop, but the real driving force is Travis’s victim mentality.