First of all, Joshua's absolutely correct in suggesting that you have the PCs get tricked into engaging in a little bit of piracy of their own. Beautiful.
Second, if the pirate is trying to pass himself off as legit, he should have a title (legitimate, or not), say...Tariff Inspector, Sheriff, or somesuch.
...And, just for fun, when the PCs come back with the stolen pig, maybe he could implement/arrest them, probably because the farmers came to him for help and he realizes that he can't possibly accept the pig from the PCs and continue to look like a legitimate non-pirate. So he passes it off as an unwarranted attack by the strangers in town.
By this time, the PCs should probably hate him, but if it's done properly, he'll have all of the innocent villagers on his side.
And all this trouble over a pig.
One more thing: later, in secret, he may give them the information that they want and let them go on the condition that they leave town quietly and never come back. If the PCs try to kill him on the spot, he can have the aid of the villagers in little time, claiming that the pig-stealing PCs are attempting an escape.