Applying common law here would be the dumbest thing humanity could do. She is basically the ONLY line of defense against this threat and animes/mangas are mainly designed, that magical girls are the only ones being able to destroy these "alien" threats. They basically destroyed their line of defence and instead of 100, all of living beings within a distance of the monsters reach die. Make this number from 100k to basically all of humanity.
Even if its the girls fault, if humanity acts this way, they should all perish, because they are dumb.