I think some people have missed that he said that if don't do it, one of your loved ones will die. So either way someones going to die, just one way blood will be on your hands from the direct act of murder, and the other blood will be on your hands indirectly from not acting. However can you trust this guy whos paying you AND taking your loved ones hostage? No. So I'd try and save them, even if it meant killing him. Then I'd take all his money and give it to various charities, keeping little to non of it for myself.
An interesting scenario, that many have snorted in disbelief and called people lunatics for declaring, would any of you be able to kill if God himself brought you to him and told you to? No one would believe you if you told them, and you'd be named a religious extremist and be treat as any other terrorist.