The weird part is how folks want "city watch in a D&D setting" to be the same thing as "a police force in the real world." They can be so much better than that. It's fantasy, after all!
To handle sticky roleplaying situations like "violence, assault, and murder" in my games, I take a page out of Matt Mercer's book and ask, "How do you want to do this?"
Let's say the party is attacked in town, by humans. Without any reliable witnesses, this typically ends with the winner being arrested for murder...the classic DM "gotcha" trap. I'd like to avoid that trap, because it's dull, predictable, and annoying. So when the muggers/pickpockets/assassins/whatever are all reduced to 0 hit points, I announce that the enemy is defeated...and then I ask the players to tell me what "defeated" looks like. It's just a fancy way of asking "HDYWtDT?"
So the players tell me what the defeated enemy looks like. Maybe "defeated" means they are all dead, sure. Or maybe "defeated" means they are all tied up, Scooby-Do style. Maybe they're all unconscious with stars circling around their heads, maybe they were routed and forced to flee, maybe the party releases them with a message for their leader ("you tell 'em we're coming, and hell's coming with us!") Whatever. The players have full control over the consequences (within reason), so there's no "gotcha" nonsense, no need to track lethal vs. nonlethal damage, no hurt feelings.