In Chapter 1, why do the boys drop the rock from the mountain?

The rock is dislodged in Chapter Eleven. I believe that Roger's intent was to frighten and intimidate Piggy and Ralph when he sent the rock crashing down the mountain, an act that led to Piggy's death. None-the-less, Roger's reaction to killing Piggy is so emotional (he's happy) that we have to wonder if that was his plan.