In Lord of the Flies, the relationship between Roger and Piggy is one of predator and prey. Roger, described once as being "furtive," is a boy who delights in the suffering of others. Roger is a classic bully. He derives enjoyment and a feeling of power from the suffering of others, like when he throws rocks at Henry or destroys the sandcastles of the little boys.
Piggy, easily one of the weakest boys on the island due to his short-sightedness and overall out-of-shape physique, becomes a natural target for Roger. If a bully such as Roger likes to prey on the weakest individuals in the camp, then Piggy would be his largest target (figuratively and literally, unfortunately). Moreover, Piggy's alliance to Ralph clearly sets him against Roger and Jack. Roger, as Jack's evil henchman, takes great delight in ending Piggy's life.