Why do Ralph, Piggy, and Samneric lie about their part in Simon's death, or use the darkness is an excuse?
In my opinion, they are doing this because they have realized just what a horrible thing it is that they have done. As Ralph says, "I'm frightened. Of us."
At the start of Chapter 11, Piggy and Ralph are talking about what happened the night before -- how Simon got killed. Ralph, alone among the others, is willing to admit what happened. But Piggy is trying hard to call it something else. It's like he's begging Ralph not to call it murder. From that, and from Samneric's reaction, it looks to me like they have guilty consciences but they (unlike Ralph) are not able to face up to their guilt.