The climax is at the end of the second scene where the murder of the boss takes place. If you have watched the play or read the text, pay close attention to the stage directions and what happens while Mr. Zero's boss is dying. There is in fact a climatic staging of music that is supposed to reflect events in the world. The audience is supposed to hear sounds that bring armistice day to mind (11/11/1918-it ended WW I) among other things. But there is also the sound of wind, waves, carnival, a train whistle etc....a barrage of sounds that are perhaps designed to dwarf human existence. Since the play was written in the 20's and it commonly considered one of the first works of Expressionism of American art, it is characterized by a sort of hopelessness and despair of the post-war existence. Even though the 1920's were a time of prosperity America, many artists did not buy into the sentiment that everything was okay and the Expressionists in particular held onto to the idea that art reflected the emotional experience, or perhaps pain, of modern life. It is this pain of modernity, the idea of being replaced, that makes Zero kill his boss.