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.
A self-righteous person quickly appears on the forum and starts pointing out minor rule violations.
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.