A quick examination of the pig's cage will reveal bones and skulls littering the ground everywhere.
As Vaas executes hostages in the distance, it becomes grimly obvious why the pigs are kept around.
The pirates feed the pigs the remains of those who they kill, either for ransoms unmet or simply for lacking in purpose anymore.
But for a group of deranged killers, having a few pet pigs around the encampment is a smart decision.
Pigs are said to eat anything thrown in their cage, even bones. The digestive system of a pig can dissolve even the coarse material of human bone, removing all evidence of the crime and saving the pirates a good deal of cleanup.