In Fahrenheit 451, why does Mildred betray Montag?
After Montag's meltdown, where he reads to guests from a book, Mildred calls the alarm, bringing Montag to his own house with Chief Beatty. He tries, for the last time, to communicate with her, but she rebuffs him:
She shoved the valise in the waiting beetle, climbed in, and sat mumbling, "Poor family, poor family, oh everything gone, everything, everything gone now...."
(Bradbury, Fahrenheit 451, Google Books)
Mildred has been so conditioned to accept the television screens as her "family" that she is in shock; her house with its three wall-screens is going to be burned, and she believes this is a terrible loss. For Mildred, her comfortable lifestyle, living inside the status quo, is proper and enough to live for; she cannot understand Montag's obsession with books, because all she wants out of life is the meaningless emotional responses that the television programs create in her. With Montag's books -- and probably Montag himself -- out of the way, she can find another "family" on television and stop worrying that she will be cast from society.
Mildred is the prototypical citizen in Bradbury's dystopian society. She is callous and superficial. Mildred does not share Montag's enthusiasm for literature and would rather watch her 'parlor walls' all day. Mildred is content living a meaningless life and being entertained by interactive television shows and Seashell radios. After Montag shows her his stash of books and reads poetry aloud in front of her friends, Mildred realizes that her lifestyle is threatened. She knows that her husband is committing a crime by hiding books and reading literature. Mildred decides to call the authorities on her husband because she does not want to be involved in Montag's precarious lifestyle. Since Mildred does not love Montag, she reports him without hesitation and can only think of her possessions as she drives away. Mildred is not loyal to her husband and simply wishes to live a mundane life which was threatened by Montag's recent escapades.