Why do you think Montag lights his own house on fire?

Faber forced Montag to torch his own home.

"I want you to do this job all by your lonesome, Montag. Not with kerosene and a match, but piecework, with a flamethrower. Your house, your clean-up."

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"Ready." Montag snapped the safety-catch on the flamethrower.
"Fire!"
A great nuzzling gout of flame leapt out to lap at the books and knock them against the wall.