Not by playing to their decency or humanity but by emphasizing how much more useful he is to them alive, as a mouthpiece for Palestinian terrorism. They sure know their audience.
Update: “Mustafa Wafi, a senior Palestinian security official, said the kidnapping was unusual because no demands have been made. ‘This is the first time kidnappers haven’t identified themselves or their demands,’ Wafi said. ‘Usually kidnappers announce who they are, sometimes as soon as they kidnap.'”