The system of Bounty of a notorious pirate is started by marines of Onepiece world. The world government when realized that Pirates are becoming stronger day by day, they exploited their weakness of greed for money and introduced the bounty system. Soon among the pirates, hunting each other for the sake of money and power began. The Pirate hunters were feared among the Pirates as well as World government. I think Rocks-Pirates also gathered for this money making scheme. Their motivation was same as their Captain Rocks D. Xebec, to make money by hunting strongest Pirates of their era. May be they were using this money to buy weapons or powerful devilfruits from underworld. Black Beard's crew is also a bountyhunter group. I think Teach's motivation is same as Xebec. We all know that zoro was initially a pirate hunter also.
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