This is one of my favorite books. I read it about 5 years ago for the first time and was shocked at how good it was for how much I hated everyone in it. To this day when I give my son a Coke I think about that passage in the supermarket when they find one of the last cans and he shares it with him, knowing that it might be one of the only good things he can give his son.
That being said, the scariest passage in the book for me was when the Man and the Boy were walking through the forest fire remains and realized that they were being followed. So they stopped and hid and let the group behind overtake them. When they did, they saw a group of four: three men, and a pregnant woman.
The next day they started ahead and came upon the group's campsite which had been hastily deserted at their approach. On the fire was the infant which had been gutted and was being turned on a spit.
That book was fucked up and that part has always stuck with me. How absolutely terrible that is, not because of what they did, but because I believe it could happen.