Why do you think Ralph doesn't really expect to encounter the beast?
In the book Lord of the Flies, Sam and Eric believe they have seen a beast. They describe the beast and the other characters respond differently. Jack wants to immediately hunt for the beast. Piggy wants to hide and make sure everyone stays on the platform. While Ralph's response is at first fearful, his is eventually the most reasonable. He understands that staying on the platform will not work, because they will run out of food and water. He wants to "think" and figure out the best way to solve the problem. Eventually they do decide to go after the beast. During the trip, Simon tells Ralph that he doesn't believe the beast is real. Ralph eventually realizes that "he did not expect to meet any beast and didn't know what he would do about it if he did." With his calm thought he's finally put the pieces together and realized they won't encounter a beast, because there isn't one. The beast is only that which the boys fear, and doesn't have a basis in reality. He realizes the beast is the fear that is within them, not an actual "real" beast.
In the beginning, he does believe like the other boys that there is a beast. In fact, he sees something sitting at the top of the mountain and believes it is a beast. Later on, right before Simon's death, Simon yells out about a dead man on a hill, and Ralph hears what he says. Ralph begins to envision the figure he saw that time on the mountain. So, Ralph doesn't feel either way about the beast. He doesn't quite believe it but he also is led astray by the other boys since they're always scared that there is a beast. In the end of the book, however, the other boys hunt him down and he feels like he is the prey and they are the predators. He feels like the boys are beasts trying to kill him. So I think Ralph doesn't expect to encounter the beast because Piggy, his companion, is always telling him that there is no beast and he starts believing that. Of course, other boys make his mind go hazy and he is stuck in the middle of believing in a beast and not believing there is one. But Piggy makes solid points, such as the existence of technology, as to why a beast cannot exist, and I think for this reason, Ralph doesn't believe he would encounter a beast. But in the end, during the hunt, he does encounter the beast---the darkness in one's heart.