If I really thought I'd identified a murderer then I'd likely go for an anonymous tip-off. An adult victim of abuse can report it or walk, generally speaking. If a child was involved there are set protocols for reporting which supercede the patient's right to confidentiality - ditto vulnerable adults. If they were just wanted by the bobbies for being an anti-social toe-rag - so what? If I reported all of those I came across I'd be at it several times a week.