In America, automatically sue them civilly by the State for lost wages, mercenary healthcare costs, any permanent impairment risks; everything.
Your "good Samaritan with a gun" (read: dude who failed his cop exam for reasons) needs to get some insurance or his life will get very different if he maims someone with his civilian-level firearms training.
Fuck it. Charge the vigilantes with a crime if they even draw their firearm (manacing) or fire it (noise/assault/threatening/attempted murder) so they Get It.