To understand Montag's reaction it is important to note his meeting with Clarisse. Clarisse had asked, "Are you happy?" and he was startled, and started to think that perhaps he wasn't. Meeting Clarisse had really upset his normal way of thinking; he was left feeling unsettled, questioning his happiness, and wondering why he all of a sudden felt old and tired. Her mere presence was so refreshing and alive, that he felt his own life drab and empty in comparison.
When he finds Millie, first of all, he is not surprised:
"An instant before his foot hit the object on the floor he knew he would hit such an object."
He was not surprised to find her empty pill bottle. This indicates that he is, at some level, aware of Mildred's unhappiness, and a suicide attempt is not a surprising thing for him. But still, as he realizes what has happened, Bradbury describes his feelings more symbolically through the jet bombers that scream over the city. Their presence has a physical impact on him, but that physical impact symbolizes Montag's reaction: "Montag was cut in half. He felt his chest chopped down and split apart. The jet bombers...did all the screaming for him." He stands there and lets the shock wash over him, and is almost numb; he calls 911 and just stands there, "his lips moving and moving." Later, he is sad and wishes that the machine could have "taken her mind along to the dry cleaner's" along with cleaning her blood. He is sad that she is so sad, and that there are so many sad people around him. He realizes that since he met Clarisse and discovered Mildred,
"the world had melted down and sprung up in a new and colorless form."
He feels like everything that he used to know is wrong now, and empty, devoid of life and happiness. It started with Clarisse upsetting his viewpoint, and was confirmed when he found Mildred almost dead.