When Beatty talks to Montag, what is his explanation for what happened to Clarisse and her family?

Beatty tells Montag that Clarisse and her family were odd and dangerous.

Clarisse McClellan is a neighbor of Montag’s and she makes a big impression on him.  She teaches him to look at the world differently.  She does this by asking him if he is happy.  Clarisse is not like the other people in Montag’s world.  She is able to slow down and appreciate the world around her.

Was it only an hour ago, Clarisse McClellan in the street, and him coming in, and the dark room and his foot kicking the little crystal bottle? Only an hour, but the world had melted down and sprung up in a new and colourless form. (Part 1)

Although Clarisse turns his world upside down, Montag loses track of her.  At first she is there sitting outsider her house every day for a week.  Then she disappears.  He has no idea what happened to her. His life feels empty.  Then one day Mildred tells him that she is “gone.”  She has been run over by one of the many speeding cars (“beetles”).  Mildred never mentioned it.

The fact that a girl’s death makes so little impact is disturbing enough, but Montag also misses the role in his life that Clarisse played. She made him question himself, and through that he became alive.

Montag mentions to Beatty that the “girl next door" was killed.  Beatty also dismisses the act.  He says that Clarisse’s family was odd.

“…She was a time bomb. The family had been feeding her subconscious .... She didn't want to know how a thing was done, but why. … You ask Why to a lot of things and you wind up very unhappy indeed, if you keep at it. The poor girl's better off dead." (Part 1)

Beatty says that “queer ones” like Clarisse and her family do not happen often, fortunately.  Beatty would not consider a bunch of kids running over a girl like Clarisse as a bad thing.  That is just what kids do—Clarisse said it herself, when she explained to Montag the depravity of kids her age.  Montag is upset by Clarisse’s disappearance and her death, but to Beatty it is just an everyday occurrence.

In order to keep a population docile, you focus on entertainment to the detriment of thinking.  In Montag’s society there are no intellectual pursuits.  People watch television, and books are illegal.  Reading is against the law, but killing is not.  Their priorities are all out of whack.  They have lost touch with their humanity.