It's quite simple. There are reasons when you can, and reasons when you must, breach patient confidentiality. One reason you must breach confidentiality is in the interest of public safety, eg if you had reason to believe that a patient was going to kill his attacker, or if a patient was a potential terrorist, for example. If you are asked by a police officer to assist you with enquiries regarding a serious crime (murder, rape) then you must help them to the best of your abilities. However, there is no requirement to breach confidentiality, just because a patient tells you he is wanted for the police.