Zeus' primary fear of giving man fire was that he didn't want man to be in equal standings with the gods. Zeus believed that fire, and its mystery, was one of the elements that allowed humans to stand in reverence of the gods. His belief was that as long as human remained in "darkness" (fire= light), they would be submissive to the gods' will. He believed that giving man fire would cause the gods to have lost some of their "lustre" in the eyes of man. In some respects, he was right, for once Prometheus gave fire to man, he also told them how to trick Zeus using fire and animal sacrifice. For his part, Zeus did not take too kindly to be ridiculed and punished both man and Prometheus in a very swift and brutal manner.
When Prometheus created mankind, he only cared about the men. Zeus was really upset, and feared that mankind would become too strong, and even warned him not to give mankind the power of fire. When Prometheus refused to listen, Zeus was enraged, and stole fire from mankind. He thought that men did not deserve to live in light, but should live in eternal darkness.