The pirates have already vowed to specifically target American and French (because the French recently pulled a similar rescue for some of their citizens, though one hostage lost their life in that one) nationals for "revenge" killing.
However, something tells me that if an American hostage is killed outright rather than used as a bargaining chip, which is the equivalent of these pirates actually becoming terrorists, the Somalis are going to end up with cruise missiles landing on their shit, and US warships shooting first and asking questions later on any aggressive acting Somalian ships.
Personally, I'd like to see some
phalanx auto-guns retasked to target small boats rather than incoming missiles, and just bolt a few of them to US ships in the area. The captain pushes the panic button and the pirates' suddenly have to deal with a rope stream of lead raining down on them. A few encounters like that and all the cargo ships will have to do is bolt a few hot water heaters to the deck and watch the pirates steer clear.