I don't think something like the unwritten rules would really come about naturally

I think they could, it seems like the most logical answer to the question of what to do when villains want to also defend where they live against the giant horror beasts & join the fight, also post-endbringer fight would be a really good opportunity to nab a few villains or end a few heroes but you really couldn't do that after the villains just helped buy enough time for the golden idiot to rock up & the villains couldn't take that opportunity without serious repercussions like kill orders

So just enact a temporary amnesty where the villains get to go home if they fight the horror beast