The incident you refer to takes place in chapter 4. Maurice and Roger have just come off duty watching the fire and encounter some of the little boys playing on the beach. The two bigger boys kick over the littler boys sand castles, laughing as they did so. This sort of action is typical bully action where bigger and physically stronger people try to overpower smaller, weaker people. It's also typical behavior among countries: larger, more powerful countries take over and control smaller countries. This is the sort of evil and savage behavior that Golding felt all people were capable of. He developed much of his dim view of manking because of his service during World War II when he saw people and countries commit these types of acts. Maurice, in this scene, also kicks sand in the face of Percival. Another, more personal, attack. Maurice, however, after he kicked the sand, had a feeling of remorse and turns away. There is still some civility left in Maurice. He hasn't become savage yet and the rules of society taught him that such behaviors were not acceptable. He is on his way to savagery, but feeling guilty about being a bully shows he isn't there yet.