Who did Montag think killed Clarisse?
The answer to this can be found in Part 3 of the book as Montag is running away after he has killed Captain Beatty. He wonders if maybe some kids out for a joyride in a car were the ones who killed Clarisse.
As Guy runs away after killing Beatty, he has to cross a wide street. As he crosses, he is almost run over by some kids in a car. It is apparently a common thing for joyriders to do -- to try to run someone over if they have the chance. Montag realizes that they are just randomly trying to kill him for fun and that they don't know that he is a wanted man. When he realizes that -- that they are just being randomly murderous -- he wonders if they were the ones who killed Clarisse.