I think Montag scares Mildred with the change in his personality and hiding of books in their home. When he reads the poem, “Dover Beach,” to her and her friends, she realizes that Montag is breaking the law, and she can also get in trouble for allowing him to have books. Mildred’s friends are the first to call in the alarm on Montag, and Mildred probably feels the pressure to do so as well. Mildred is a product of her environment; she has fallen victim to the mind-numbing society in which she lives. She is obsessed with the soap operas on her wall-size TV’s and is unconsciously so depressed that she overdoses on sleeping medication. Mildred is afraid, not only of Montag but of society who searches out rebels like Montag who are becoming free-thinking individuals. In addition, Montag and Mildred’s marriage is loveless. They have grown apart over the years; and therefore, it isn’t particularly hard for her to turn him in, pack her bags, and run before the house is burned down.
I think that there are a couple of reasons for this.
First, I think that Mildred truly buys into the ideas of the society. She loves things like the "families" in the parlour walls. She does not want to think. She just wants the life that society says she should have. So she does not want Montag to go against society and hates him for doing it.
Second, I think that she personally does not want to live with someone who holds views like his. She does not want him to pester her about reading like he has started to do.