vigilante
ˌvɪdʒɪəˈlænti noun
1 someone who illegally punishes criminals and tries to prevent crime, usually because they think the police are not doing this effectively 民间护法者,义务警察〔指私自非法惩治罪犯、试图防止犯罪的人,常常是因为他们认为警方办事不力〕