Character Analysis
This passenger has some serious issues. He pays Travis to park the cab outside an apartment building, where he rants about how his wife is upstairs in one of the apartments having an affair—with an African-American guy (the passenger uses the N-word). They can see her silhouette in one of the windows.
The guy keeps ranting, saying that he's going to use a .44 Magnum pistol to blow her face off. He laughs about this, saying that Travis probably thinks he's pretty sick, while Travis just sits there in silence. To Travis' mind, this is probably just more evidence about how screwed-up the world is… and how he needs to take matters into his own hands.