Why do you think golding (the author) plotted the story so that the boys would go up the mountain in the dark? lord of the flies?

It is part of the savagery of the story. It is the vast contrast in light and dark, right and wrong, safe and unsafe. Each boy wants control, so to look for the beast after dark up the mountain shows the extremes the boys will go to for this power, and even the survival of the fittest.