Bradley Manning was a member of an organization (the military) which has specific codes and rules that must be followed in order to continue to work with the organization. Bradley Manning broke these rules and is subject to the punishment that comes with them. He behaved dishonorably and his discharge should reflect that.
Yes, Bradley Manning should be dishonorably discharged as when he released those cables without first knowing what was in them, he endangered the lives of many, many people. He may have had the best of intentions at heart, but the fact remains that he endangered lives and did not think about the consequences of his actions.