well depends on your definition. In short it means attempting to make justice without authorization.

In the case of doing legal research, accessing things that are happening in public, and turning them in to authorities (be it community wise, or law enforcement. I would say very few have any opposition to.

Where it gets far greyer is when you are doing things that are illegal or imoral in and of themselves without evidence of what you are dealing with.

IE say you hack someone’s computer to discover they have been doing illegal actions etc… In the case of the guilty person most people are good with that… but it begs the question, did you hack a bunch of innocent people, to find that one guilty person. How did you know they were guilty etc…

and then of course the more absolute extreme ones, when you apply the punishment yourself in spite of that being a crime in and of itself. Say you beat up, kill, kidnap, destroy proporty etc… That’s generally frowned upon, except in instances where most people can agree on the horror of the crime, and completely lack any faith in the legal channels to appropriately enforce it even when the evidence is right in front of their faces. IE why fictional superheros like batman are popular (and why making him popular mostly involves the story implying that gotham’s police are either incompetent or too corrupt to enforce the law), as well as say in the real world people like Luigi are loved.