Guy Montag is the perfect citizen of his society at the beginning of the book. He is a fireman who doesn't put out fires but starts them. He finds pleasure in burning things.
"It was a special pleasure to see things eaten, to see things blackened and changed." (pg 3)
The brass nozzle he holds is filled with kerosene, and he visualizes himself as the conductor of a symphony, a symphony that will burn and tear down the "charcoal ruins of history". He is a man who makes beautiful music with his kerosene and igniter. He flicks the igniter and sets everything on fire, fantasizing about putting a marshmallow on a stick and roasting it while the ashen pages of books float around him.
He grins a "fierce" grin -- one of pleasure. Similar to a pyromaniac, he enjoys a good fire. He sees himself in the mirror as a "minstrel man", a man who creates music and song from events such as this, and he knows he will go to sleep with that same grin on his face. He always had that grin after a fire. It never went away as long as he could remember.