Why does Clarisse have to die in Fahrenheit 451? Why did the author make Clarisse die?
Clarisse doesn't have to die in this novel. There are several reasons, however, why it makes sense for Clarisse to die.
First, Clarisse functions as a plot device: she is one of the "drops" of water falling on Montag (along with Mildred's suicide attempt) that leads him to begin questioning the way he has been living his life. Once she has fulfilled that function, she in some ways becomes an encumbrance in the novel: after all, an older, married man like Montag can only have a friendship with an underage girl for so long without raising uncomfortable questions, no matter how pure his motives might be. Killing Clarisse gets rid of the problem of a relationship that really can't develop.
Second, killing Clarisse illustrates that this is a society in which people like her can't expect to survive. The society doesn't want thinking, questioning, vitally alive people like Clarisse rocking the boat and raising uncomfortable issues about the culture in which they live. As Beatty says:
The girl? She was a time bomb. ... She didn't want to know how a thing was done, but why. That can be embarrassing. 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.