When the slaver was first removed from Sencliff it made sense to have the Abolitionist there to sort of explain what had transpired even if it was sudden and didn't have a lot of buildup. Similarly, removing the slaver from Sencliff made sense imo because it was strange to have the fortress island presented as a pirate haven... Where npc pirates would enslave their fellow pirates.
That said, now it's weird that the abolitionist is there because few characters let alone players are old enough to remember how he ended up there and it's weird that pirates who are largely cooperating out of a shared interest in criminal enterprise would have a strong general stance in either direction on the subject.