I am curious if, during the specific time period of the transatlantic slave trade, there were pirates who preferentially targeted slave ships. In particular, I would like to know if during this time period, there were any accounts of pirates who commandeered a vessel and freed its captives.
The best resource for this would be the Trans-Atlantic Slave Voyage Database, which is freely accessible online. If you go through the filters on the left of the page in the "General Variables" section, you can select filters for pirates seizing slave ships. After a quick search, I found 121 slave ships that were captured by pirates or privateers. A rough outline of the data shows that most of the seizures took place in the eighteenth century (79), and that about half of the ships had slaves on board at the time of seizure. You probably would want to look at the voyages individually to see if there are any instances where pirates freed the slaves on their own accord. For almost all the voyages I found, the pirates sold the slaves in the Americas.
Of the 50 ships where we know their main port of sale, here is a rough breakdown:
Spanish Americas: 13 ships
US/British Colonies: 7 ships
French Caribbean: 23 ships
Brazil: 3 ships
Other: 4 ships
The Voyage ID's in question are: 20770 21390 37014 11963 30090 30115 5050 24810 25553 25684 25761 25153 32474 36225 36237 35234 34190 34260 34720 48909 46917 46919 41553 41554 41572 42832 42903 39028 41885 41894 42177 46708 37029 36192 549 33118 16250 16289 18269 17120 10141 30705 30141 30171 27176 27178 27182 26099 26332 27001 24505 24508 24532 82704 90428 92593 78231 77245 77254 77685 77745 76527 75996 76017 75276 76400
As a related question, was it really possible for pirates to preferentially target ships based on cargo at all? I know the trade lanes were reasonably well-established, to the extent that one might know that, say, a given Portugese ship coming toward Brazil from Africa would be likely to be carrying slaves, or a given English ship sailing on the route from Jamaica to England might be carrying rum, but how reliably would they be able to prey on those trade lanes? Would it be mostly catch-and-release until they found a particularly valuable cargo, or would they simply seize any ship of the correct nationality that they stumbled across and take anything of value?