Must or not

One family went on a trip to Nablus, for which they hired a security guard with a gun, and agreed to pay him at the end of the tour. At the end of the tour, it was discovered that the security guard did not have a magazine in his rifle, so if they had been shot at, they would have died. The family argued that since they could not do anything if they had been shot, they did not have to pay. The security guard claimed that the gun created a certain deterrent and therefore they should pay him. Who is right and why?