While Montag's curiosity for books has been sparked by his relationship with Clarisse, he is still a faithful fireman. He has burned houses and people in the past, secure in the knowledge that this is his job, sanctioned by the government. However, with his resolve weakened by Clarisse, and his own desire to read books stronger than ever, he is mentally broken by the suicide of an old woman, which shows him that even though life is no longer valued, some things are worth dying for:
...Montag... stood near the woman. "You're not leaving her here?" he protested.
The woman replied quietly, "I want to stay here."
Beatty flicked his fingers to spark the kerosene.
He was too late. Montag gasped.
The woman on the porch reached out with contempt for them all, and struck the kitchen match against the railing.
(Bradbury, Fahrenheit 451, Google Books)
Her desire to destroy herself and her books rather than allow the government to take them from her ignites a fever in Montag, who cannot grasp or understand how an inanimate object could cause that sort of love and devotion. He is struck by her quiet strength, refusing to be bullied or controlled, and by her contempt, which is appropriate for his destructive actions. Until this point, he had no doubts about his own work, even when he wondered if books were really so bad; at the beginning of the novel, he loves to burn things, and after this, he has trouble reconciling his work with his new ideas.