I know this isn't a superhero/comic board but this overlaps... this is why I HATE super hero registration stories. We don't need whole knew laws and concepts. We HAVE good Samaritan and Vigilante laws and concepts... and those can be enforced (bare with me a bit)There are actually real-world laws related to this sort of thing; "Good Samaritan" laws, for example, specifically exculpate people for "ordinary negligence" (that is, for example, a failure to conduct CPR by proper current procedure, but still conducting CPR to try to save someone's life), even if that ordinary negligence might have been a contributing factor to someone's death. They do not exculpate "gross negligence," which usually requires the demonstration of explicitly willful, wanton dereliction of duty, possibly even malicious intent.
you went out (mask/costume or not) and stopped an armed robbery in progress, you are a hero. You save a life (CPR, Magic, what ever) and cause minor damage in process... good Smartian laws cover both. You track down someone guilty of a crime and beat them to a pulp or worse... this is where vigilante laws come in, and you are most likely on the wrong side (but cops and prosecutors can look the other way as long as public opinion is on your side)
In D&D kingdoms that adventurers are the norm (even if we say less then 1% of population if the kingdom is 1/2 a million that still leaves what 5kish so if we say they are half that and most are in groups then we still have somewhere around a thousand groups) they can (but don't have to) have something like this.
over the years most games hand wave this... but think about it. A well armed (and most likely wielding magic close to artillery's level) of unaligned (like not alligned to kingdom or town watch not alignment mechanic) mercanaries walk into town... and they find and kill a cult, and they help protect from an orc or undead attack... how does town guard handle that?
(BTW I always joke 'carefully and diplomaticly so they don't get fireballed')