Muslim fanatics
Most hostage takers take the path of negotiation simply because a dead hostage is worthless. Even if their initial inflated demands aren't met its usually possible for them to extract some kind of return for the victims life..even if its only money.
Its a unique feature of these muslim militants that life is of no value to them. Death is more glorious, and gruesome death serves their purpose even better because it strikes at the core of their desire to terrorise, and is consisatent with their twisted religious beliefs.
They never intended to release Mr.Johnson, and his gruesome murder (like that of Nick Bergs) serves to increase their profile in the fundamentalist Islamic world, and to scare westerners out of SA, which is the first step to their Islamic revolution in that country.