Why do Ralph pigg and samneric lie about their part in Simons death or use the darkness as an excuse?

The rationalization is undertaken mainly by Piggy. Ralph is only too aware they the boys collectively murdered Simon is a frenzied attack and he is deeply upset and filled with remorse. Piggy tries to down play their role insisting that it was a combination of the storm, the dance and even Simon's own fault, for wandering around in the dark, which led to his death. Piggy's advice is to simply forget about it as it can't be changed and thinking about it serves no purpose.