While escaping from the second Mechanical Hound, Montag is almost hit by a passing car; he is saved by falling flat, and the car swerves, but then turns around and drives back, deliberately trying to run him down.
A carful of children, all ages, God knew, from twelve to sixteen... had seen a man, a very extraordinary sight, a man strolling, a rarity, and simply said, "Let's get him..."
This is foreshadowed near the beginning of the novel, when Clarisse mentions that people are so emotionally-deadened that they look for violent outlets for the feelings they don't understand:
"They run us so ragged by the end of the day we can't do anything but... go out in the cars and race on the streets, trying to see how close you can get to lamp-posts, playing 'chicken...'"
(Bradbury, Fahrenheit 451, Google Books)
This shows the way that human life has become unimportant, something to be taken at will, and nobody cares when someone dies, or even who killed them. Montag's job as a fireman includes burning dissidents if they don't comply; people think nothing of it. The husband of a friend of his wife commits suicide; her friends laugh about it as if it makes no difference. The wilful destruction of the teenagers shows how they are indoctrinated early that their lives are meaningless compared to the larger society; Montag, discovering his individuality, slowly creates the moral framework to be terrified of this future.