Fairly interesting read, but I think it has missed potential. It fails to consider the effects magic would have on the laws of a society, and also the impact that monsters abound would have over the common folk. In a world where monsters do exist, the likely scenario would be one where small villages or settlements wouldn't exist, and people would flock together to bigger towns or cities, behind the protection of walls. With crowds, comes business, and then the need to travel through dangerous wilderness, and with it, the job of a sword-for-hire, and, the ubiquitous presence of fortune-seekers, adventurers, lacking a place of presence or belonging. And then it would all depend on how the population of a certain place sees these adventurers, trust or distrust built upon decades of dealings and the actions of such people (adventurers).

Another point to consider, is how magic could develop to aid in judicial matters, maybe spells specifically crafted to tell lies from truth, or to trace the scents left on a dead person to their killer...

And I guess this is why most editions of D&D do not really concern themselves with this topic, it's a bit too hard to point out exactly what would be the laws and such. However, I do like the thought exercise, and you can take it a step further, and write down a small set of simple laws and societal taboos for each region that could be relevant and meaningful for your game.