self-righteous person; self-appointed rule enforcer
Internet slang, derogatory. Refers to someone who aggressively pushes their own sense of justice or rules on others, often in online communities.
掲示板にすぐ正義マンが現れて、細かいルール違反を指摘し始める。
A self-righteous person quickly appears on the forum and starts pointing out minor rule violations.
あの正義マンは自分
That self-proclaimed enforcer is annoying because they push their own opinions on you.
Compound of 正義 (justice) and マン (man), likely modeled after superhero names. The exact origin is uncertain, but it emerged as internet slang.